Liberal Democrat leader Nick Clegg has said he would not enter into another coalition with the Conservatives if they insisted on their proposal of £12bn welfare cuts.
The deputy prime minister told the BBC the Conservative approach was "downright unfair".
The Tories aim to cut £12bn from the welfare budget by 2017-18.
Chancellor George Osborne has said details will be set out in a post-election spending review.
The Lib Dems have said they would cut less than the Tories and borrow less than Labour as part of their plan to eliminate the deficit by 2017-18.
In an interview with BBC Newsnight presenter Evan Davis, Mr Clegg said he "could not countenance" recommending to his party they join a coalition with "a Labour Party that's not serious about balancing the books".
In the same way, he said, he would not recommend a coalition with the Conservatives if they "insist on a plan which is... a remarkable departure from what we've done in this coalition where we've actually asked those with the broadest shoulders to pay more through the tax system to balance the books".
Mr Clegg said he was "no slouch" when it comes to welfare savings, having set out plans to cut £3bn from the budget.
But he criticised the Conservatives' planned time scale and said the Tories were "asking the poorest to make those additional sacrifices at the same time as saying to the richest that they do not need to pay an extra penny to balance the books - that's downright unfair".
The Lib Dem leader also said the coalition government had not been "right wing", pointing to measures including raising the income tax threshold and the pupil premium to support poorer schoolchildren.
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