David Cameron faces another bruising showdown over Europe with Tory backbenchers and Labour joining forces to demand a tougher line in budget negotiations.
The Prime Minister and Chancellor are said to have been holding personal meetings with wavering MPs as government whips struggle to minimise a prospective rebellion.
The EU's funding package for the next seven years is due to be decided at a summit next month, with Mr Cameron threatening to veto any increase above inflation - currently around 2%.
However, dozens of Conservatives have signed a Commons amendment urging him to hold out for a real-terms cut.
Labour has also said the budget should be reduced in recognition of the problems faced by member states, raising the possibility of an humiliating defeat for Mr Cameron.
Commons Speaker John Bercow is expected to confirm that the amendment will be put to a vote, after the Opposition signalled it would not table a rival text.
Mr Cameron could also be challenged on the controversial issue at the weekly Prime Minister's Questions session.
Although the result is not binding on the government, it would be awkward for the premier to defy the will of the House.
Tory rebels said they believed 40 to 60 of their colleagues were ready to back the amendment, including Mark Reckless, Mark Pritchard, Zac Goldsmith, Bill Cash, John Redwood, Bernard Jenkin and Peter Bone. Many more could abstain.
However, some rebels questioned whether all Labour MPs would turn up to support the call.
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