Former Lib Dem minister Sarah Teather has said she will rebel against the coalition and vote against plans to cut benefit payments.
On Tuesday MPs will vote on the government's proposal to restrict the rise in benefit payments to a below inflation 1% - a real terms cut.
Nick Clegg has argued that the Lib Dem presence in government prevented even harsher cuts to welfare payments. However this was not good enough for Teather.
"We have a huge problem with in-work child poverty and we're only going to make this significantly worse," she said on Monday.
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."
The Brent Central MP, who faces a tough fight to hold on to her seat against Labour at the next election, lost her job as children and families minister in the September reshuffle.
Teather's announcement that she intends to rebel against the coalition came just one hour before Clegg and David Cameron were due to host a joint press conference designed to show coalition unity or purpose.
She also attacked the "setting up" of a "strivers verses scroungers" narrative that drove "envy and division".
"Families who are earning the least amount of money really struggle to be able to pay for their basics," she said.
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