A senior government adviser today warned that Labour's welfare reforms could push benefit claimants closer to poverty.

Sir Richard Tilt, the chairman of the social security advisory committee, said the government should delay changes to the benefit system because the economic downturn was making it harder for people to find work.

Tilt was speaking ahead of the introduction of new rules next week that will force lone parents with a youngest child of 12 or over to look for work when they submit a new benefit claim.

Tilt's warnings were rejected today by James Purnell, the work and pensions secretary, and his Tory shadow, Chris Grayling.

Over the next few years the government will role out changes to the welfare system intended to encourage lone parents into the job market. Lone parents with older children are gradually being transferred from income support, which is paid without conditions, to jobseeker's allowance (JSA), which is only paid to people looking for work.

JSA claimants face a 40% benefit cut if they do not actively look for a job.

Tilt, whose committee advises the government about new welfare regulations, told the BBC the reforms should be delayed because the economic climate would make it hard for people to find work. He said he did not want the reforms "falling into disrepute".

"Benefit rates are relatively low and if you are going to reduce someone's benefit for a few weeks by 40% you are pushing people much closer to poverty," he said. "Of course, the child will suffer, but it's not the child that has fallen foul of the system."

Tilt said he was concerned about the availability of childcare. He said there could be valid reasons for parents staying at home.

"It may be to do with disability or chronic illness, or in some cases it may be to do with behavioural problems," he said. "So pushing the lone [parent] in those circumstances into work may actually not be in everybody's interests."

Purnell told the BBC's Today programme: "I think it would be wrong, at a time when it may be harder for people to find work, to provide them with less help. We know that our help works; we know that the help they get from the voluntary sector, from providers and from JobCentre Plus works, it changes people's lives.

"What we require people to do is come in and take up that help and when I talk to people about it they say: why didn't you make me do this earlier because it has changed my life."

Grayling said the changes were essential. "It would be disastrous for Britain to do a U-turn on welfare reform," he said.

"It would have the effect of making poverty worse and condemning millions of people in some of our most deprived communities to endless benefit dependency.

"Right now, when the jobs market is tough, we need real action to help people who've been on benefits for a long time to make the journey back into work and not simply assume that because unemployment is rising that there's no hope for them."

The leaders of Labour's 10p tax rebels yesterday served warning on the chancellor, Alistair Darling, that he has to find at least £1bn in Monday's pre-budget report to help the 6 million people who have still not been compensated for the disastrous decision to abolish the lower tax rate earlier this year.

Frank Field and Greg Pope, the leaders of the tax rebellion earlier this year, have written to Darling, urging action. Field told the Guardian: "The Labour backbenches will not abandon the poor who have still not been compensated. It is a Rubicon they will not cross, and at a time the government has found £50bn to bail out the bankers, the Treasury can surely find £1bn to ease the resentment of our core voters."

The chancellor is expected to announce a series of business and personal tax cuts in Monday's pre-budget report, aimed at the poorest because they are most likely to spend any extra cash.

The intervention of Field and Pope will add to the pressure on the Treasury which was last night seeking to dampen speculation of a massive tax giveaway next week after the latest official figures showed a backdrop of rapidly deteriorating public finances.

In the wake of the first October deficit in 14 years, officials dismissed suggestions that the chancellor could unveil a £30bn package - worth 2% of GDP - designed to lift the economy out of recession.

The quarterly payment of corporation tax means October is normally a surplus month for the public finances, but the slowdown in the economy, the collapse of the housing market, financial turmoil and tax concessions since the budget combined to leave the state in the red by £1.4bn last month.

City analysts said the budget deficit could rise to £70bn this year and top £100bn in 2009-10 even before the extra borrowing for Monday's fiscal package was taken into account.

In their letter Field and Pope write: "We are anxious that the government's promise to do all in its power to compensate fully the losers from the abolition of the 10p rate is not only met, but kept clearly separate from other tax reductions the government may announce next Monday."

Following an unprecedented backbench rebellion, and amid signs that Brown's leadership was at risk, the Treasury hastily assembled a £2.7bn compensation package in May. Darling increased personal allowances for all basic rate taxpayers. Although only 1.1 million householders have lost out overall, this masks the fact that 6 million individuals have been losers.

Field and Pope write: "Overwhelmingly these taxpayers are on low earnings. The Institute of Fiscal Studies estimates that the greatest loss of around £112 a year are for taxpayers earning £7,755."

They accept that the complexity of the tax system means it will be impossible for the Treasury to help every group that has lost, but argue it does seem likely that most of these losers are on low earnings and fall below a threshold of £13,355 a year.

"We do hope you can give us an assurance that Monday's statement will include a measure that will recompense as many of these individuals as is practically possible. Only in this way would it be possible to draw a clear line under this wholly sorry saga."

Field claims the net cost of his proposal will be £1bn.

In other developments the employment minister, Tony McNulty, said the pre-budget report would propose a new employment programme, adding it was "a no-brainer" to revisit the closure programme for Jobcentre Plus officers. Since 2002, the government has closed nearly 500 job centres, including 40 in the last year, and cut staff by 16,000.

Ministers are also under intense pressure to use the pre-budget report to rethink its housebuilding programme after figures were released yesterday showing only an estimated 22,200 housing starts in England in the September quarter, down 33% on the previous quarter.

This decline in housebuilding levels makes the government's target of building 240,000 homes a year by 2016 look hopelessly unrealistic.

'In a recession, nobody will be interested in employing single parents." Julia, a lone mother of two children aged nine and 12, is pragmatic about her options. "Most of us will have been out of work for some time, and need hours to fit with school. That already puts us at the back of the queue. But in an economic downturn, I think it's going to be virtually impossible to get a job."

Julia's assessment of her prospects is abysmal, but fair. What is not fair is that, as of Monday, single parents with a child of an age with her eldest will be forced to find work of at least 16 hours per week, or risk a 40% benefit slash. The difficulties of securing a part-time job that dovetails with adequate childcare, even for the best-placed parents, has been well rehearsed. So this move is especially devastating, coming at a time when the government has just reneged on its commitment to flexible working while unemployment escalates.

As a former business analyst, Julia assumed that - once she had eased her children through the trauma of divorce - re-entering the workplace would be reasonably straightforward. She was wrong. After a long succession of confidence-sapping rejections and childcare-incompatible offers, she remains unemployed.

"I tried for office skills jobs - through agencies, the internet, the jobcentre and directly to banks - but none were viable. Most after-school clubs finish at 6pm and if you are working and expected to do overtime you can't get home for that. When you have older children, you can't keep an eye on what they're getting up to or be there if they're lonely. And you just can't get the holidays you need to be with them when they're not at school."

Julia's problems with inflexible working will be familiar to all parents, but are hugely exacerbated when functioning solo. And the chances of any immediate improvement are slim. Last month, new business secretary Peter Mandelson's imperative to ease the burden on businesses during recession found an early casualty in Labour's commitment to a more family friendly employment ethos.

Despite this, the process of transferring lone parents with progressively younger children off income support, which they can currently claim until their youngest child is 16, and on to jobseeker's allowance - which requires them to take work or face sanctions - looks set to continue. In less than two years' time, the age of the youngest child to shift this already vulnerable group on to far more treacherous territory will be just seven, affecting more than 300,000 families.

The new regulations do allow lone parents who cannot find "suitable, affordable or appropriate" childcare to turn down a job on these grounds. But how this will be applied in practice remains untested. And the regulations do not take into account the particular needs of parents with disabled children, or those experiencing problems at school, nor parents who have only recently separated. It is blind absurdity to compel a parent to work purely on the basis of their child's age. While the government's rhetoric around childcare is all about parental choice, these measures remove that choice, leaving the ultimate decision to the discretion of a Jobcentre Plus advisor who is manifestly not best placed to assess the needs of an individual family.

It is a further irony that this policy will severely limit lone parents' capacity to ready themselves for the employment market. Thirty per cent of single parents on benefits lack any formal qualifications. Yet a transfer to jobseeker's allowance means that many will be denied the opportunity to participate in education and training, since the benefit isn't available to those studying full-time.

Labour only started to take an interest in childcare when it surmised that one way to tackle this country's appalling level of child poverty was to get mothers into the workplace. But, despite the success of Sure Start, provision for older children - those who will be affected by the new regulations - remains poor. A recent report for the charity 4Children found that there was only one place for every 200 children aged 11-14.

While Labour is right to conclude that helping single parents back to work is the best way to ameliorate their disadvantage, it's plain these regulations alone will not achieve that end. The independent social security advisory committee recently reported its reservations about the proposals, noting "the underlying tensions between policies to promote greater personal responsibility for their children, and greater engagement in securing their health and wellbeing, and policies that may have the effect of forcing lone parents to give priority to paid employment."

So welfare-to-work has its limits: more than half of children living in poverty already have a parent in employment. And these proposals also subscribe to a middle-class agenda which assumes that all mothers - and the vast majority of single parents are women - are desperate to return to meaningful, well-paid jobs which fulfil their sense of self beyond the home. While the majority of single parents in this country are in employment, the ones who remain on benefits are by definition those who need much more than a bureaucratic kick up the arse to get them behind the till at Tesco. Credit crunch or not, it makes no sense to remove support for the people who choose to concentrate on the hard work of parenting in difficult circumstances.

Public sector unions and opposition politicians yesterday seized on figures that show a record number of claimants are waiting a fortnight to receive unemployment benefit.

The government was urged to end its closure of Jobcentres after it was revealed that the amount of unclaimed unemployment benefits was at a four-year high.

In the last quarter of the year, 66,545 people who signed up for job seekers allowance (JSA), were defined as "unprocessed".

Last night the Department for Work and Pensions said that though the backlog figure had increased, 75% of its customers were being seen within its 11.5 day target. Senior government sources said the department was watching the situation very closely but that staff were currently "coping".

In recognition of choppy times ahead it had recruited an extra 2,000 members of staff to cope with the increased burden. There were also reports that a recent round of Jobcentre cuts had been shelved.

The four-year high comes as the number of JSA claimants hit 980,900, up 150,000 on the year before.

The Liberal Democrats said the backlog meant a greater number of people could be waiting for up to a month to get their money. In the year since last December 40 Jobcentre Plus offices have been closed and yesterday unions told the Guardian members working in Jobcentre Plus had reported an increase of 20% in calls. Since 2002, 491 Jobcentre Plus offices have been closed with more closures planned.