David Cameron faces a serious challenge to his authority from Conservative eurosceptic backbenchers later, but Downing Street is insisting this evening's likely vote is effectively irrelevant.
No 10 has given backbenchers a free vote on the issue and even told ministers they can abstain from John Baron and Peter Bone's amendment to the Queen's Speech regretting the absence of an EU referendum bill.
With Labour having confirmed it will back the government in opposing the amendment there is no chance of another Commons defeat for Cameron.
But over 90 MPs have now signed the amendment and their protest will go down as a serious setback for a Conservative party leader who had hoped his Bloomberg speech in January had ended Tory turmoil over Europe until at least the next general election.
It remains to be seen whether yesterday's publication of a draft EU referendum bill rubber-stamped by Downing Street will diminish the impact of this evening's revolt. The Conservative party is hoping a Tory MP will adopt the bill after tomorrow morning's ballot for private member's bills.
"We'll have to see what happens. The prime minister has said he's relaxed about how backbenchers vote," Europe minister David Lidington told politics.co.uk in an interview yesterday.
"A referendum on Europe during the midpoint of the next parliament is Conservative party policy, though not coalition policy. So I think we would very much expect the overwhelming majority of Conservative MPs would be willing to support the private member's bill along the lines of the one published [yesterday]."
None of the Tory MPs spoken to by politics.co.uk in parliament yesterday expressed satisfaction with the current arrangement.
One said he was backing the amendment because he felt his voters expected him to be deeply eurosceptic - but that he disagreed with its "regret" wording as "ridiculous".
Another criticised Cameron for having adopted the private member's bill alternative, warning this would "dilute" the basic message that the Conservatives are the only party offering a referendum in the next parliament.
A third MP said he wanted to see a referendum - but had not yet decided whether to support the amendment or not.
Speaker John Bercow will decide whether the amendment is voted on this morning. Under Commons rules amendments to the Queen's Speech debate are permissible from the main opposition party, a second opposition party and a third, non-aligned source.
A recent Commons briefing paper explains: "Any member may table an amendment to the address, or add his or her name to an amendment tabled by any other member." It then adds: "No amendment to the address has been carried since 1924."