Nick Clegg has said that he would not enter coalition with the Conservative party again after May’s election if the party insists on introducing its proposed £12bn of social security cuts.

Speaking to the BBC, the leader of the Liberal Democrats said that, just as he could not countenance recommending to his party that it entered into a coalition with a Labour party that was not serious about balancing the books, he would not suggest it went into government with the Conservatives if they insisted on a plan that was a marked departure from what the two parties had done in the current coalition.

Whereas the Conservative party has pledged to make £12bn in social security cuts by 2017-2018, the Liberal Democrats have said they would cut £3bn.

“[The Conservatives] are asking for £12bn over two years,” said Clegg, speaking to Newsnight’s Evan Davis.

“We’ve made £20bn over five. They want to ask the poorest to make additional sacrifices while not asking the richest to pay an additional penny through the tax system to balance the books – that’s downright unfair.”

The deputy prime minister stressed that he was proud of the coalition’s record, which he said he didn’t think could be described as rightwing.

Clegg has previously avoided speaking so explicitly about policy red lines that his party would not cross in future coalition negotiations, but the front page of the Liberal Democrat manifesto is reserved for five policies that the party has said would be a priority for it.

The Lib Dem leader frequently says that the party implemented four out of five of the policies on the front of its 2010 general election manifesto in government.

George Osborne has come under pressure to explain how the Conservative party would find cuts to the welfare budget of £12bn, saying such things would be agreed as part of a wider spending review after the election.

Speaking after Osborne announced March’s budget, Paul Johnson of the Institute for Fiscal Studies said: “We have been told about no more than £2bn of the planned cuts to working-age benefits. And, remember, apparently the ‘plan’ is to have those £12bn of cuts in place by 2017-2018. It is time we knew more about what they might actually involve.”