Liberal Democrat MPs are likely to march through the voting lobbies with their Tory colleagues to support the real-terms cut in welfare benefits on Tuesday evening - but there is deep unease within the party over the measure.

In the Commons this morning, Labour's deputy leader Harriet Harman told Nick Clegg that if he voted for the Benefits Up-rating Bill, which restricts the rise in benefits and tax credits to a below inflation 1% for the next three years, he would make "millions of low income families worse off".

Harman also attacked the rhetoric surrounding the debate, depicting those out of work and claiming benefits as "people lying in bed with the curtains drawn", as "no way to talk about unemployed people".

Clegg defended his decision to sign up to the cut, arguing that Labour should explain why it voted in favour of restricting public sector pay rises to 1% but was not prepared to do the same for benefits.

"Where is she going to find the £5bn this measure would save?" the deputy prime minister asked. "Would she take it from schools? Would she take it from social care?"

Clegg has insisted that the Lib Dems were instrumental in preventing the Tories from imposing a wholesale freeze on benefits in the Autumn Statement. However, he is under pressure from party activists and some of his backbench MPs over the compromise deal reached - as well as the language used by Tory ministers to sell it to the public.

Yesterday former Lib Dem schools minister Sarah Teather announced she planned to vote against the Bill. "We have a huge problem with in-work child poverty and we're only going to make this significantly worse," she said.

Speaking on BBC Radio 4's World at One programme, Teather added: "I feel deeply anxious about the policy and I will be voting against the Bill tomorrow very reluctantly and with a very heavy heart."

South Manchester Lib Dem MP John Leech also pledged to vote against the Bill today.

"I voted against the welfare reform bill, and I find it objectionable that the Tories are using 'skivers vs strivers' rhetoric to justify a cut to seven million working families," he said.

"I strongly support raising the tax threshold for low paid workers, but this cut will wipe out much of that good work."

Teather's public opposition was welcomed by Naomi Smith, the co-chair of the left-leaning Lib Dem pressure group, the Social Liberal Forum, who told the Huffington Post UK that she would "almost certainly" want Lib Dem MPs who were not part of the government to vote against the Bill at its final reading in the Commons.

"My own view is that the welfare cuts are unacceptable when accompanied by tax cuts for the rich," she said.

"If the government so loathes state supported 'scroungers' why not start by cutting the civil list, if indeed we are supposed to all be 'in this together'.

She added: "I'm pleased that Lib Dems have curbed some of the worst excesses of this Bill, but am deeply concerned about the cumulative effect on the most vulnerable of further cuts the Tories will doubtless call for in Comprehensive Spending Review 2.0 and potential spikes in inflation.

"Those most vulnerable to price shocks are going to face real fuel and food poverty if we see further energy price and commodity price rises."

Other Lib Dem MPs have been more circumspect about how they will vote, with Cambridge MP Julian Huppert telling HuffPost UK that he would wait to hear "how the debate goes".

"I would never decide before the vote actually happens how to vote on anything. The whole point of having a debate is to listen to it," he said.