Ministers should rethink or delay plans to force lone parents, disabled people and the long term jobless into work, a senior government advisor has said.
Sir Richard Tilt said the reforms could "push people into poverty" because of rising UK unemployment.
Under the changes, from next week lone parents will be expected to seek work once their youngest child is 12 or face having their benefits cut.
Employment minister Tom McNulty said reform was more vital now than ever.
He said: "Now more than ever it is important to help people prepare for work and move closer to the labour market.
"In the 1980s and 1990s people were moved onto incapacity benefits and left to languish with no help or support to return to work.
"These changes will ensure that lone parents are ready for work whenever jobs become available."
'Closer to poverty'
Sir Richard, who is head of the social security advisory committee, said the so-called "welfare to work" reforms risked "falling into disrepute".
He called for the changes to be delayed by one or two years.
Instead, they will be able to claim Jobseekers Allowance if they are actively looking for work.
The total amount they can receive will be almost exactly the same, but anyone not complying with the new rules could face sanctions, including having their benefits cut by up to 40%.
Sir Richard said: "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.
"Of course, the child will suffer, but it's not the child that has fallen foul of the system."
Sir Richard said he was concerned about the availability of suitable, affordable childcare for lone parents affected by the changes.
And he said there was often a particular reason that the lone parent was 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 interest.
"In many cases it will be harmful and lead to further behavioural problems."
For lone parents already claiming income support, the changes will be phased in depending on the age of the youngest child.
From March 2009, parents of 14 and 15-year-olds will no longer be able to claim income support and from July 2009, the change will extend to parents of 12 and 13-year-olds.
By October 2010, lone parents with children aged seven and over will be required to look for work in order to get benefits.
According to the government, there are 1.8m lone parents in the UK, 56.3% of whom are in work.
There are 738,600 lone parents on income support, and of those, just over 100,000 have children aged 12 and above.