David Cameron could be dramatically defeated by his own party in the House of Commons over his promised EU referendum, just over a month after he secured a shock majority at the general election.
The prime minister reportedly faces a rebellion from Eurosceptic Tory MPs, who plan to back a crucial amendment to the government's EU referendum bill on 16 June.
The Conservatives want Cameron to legislate for a pre-election purdah period so local and central government will be barred from making political announcements as the EU vote looms.
The condition was enforced in the 28 days ahead of the Scottish independence referendum in 2014.
Around 50 MPs could defy the prime minister over the issue and back Sir William Cash's amendment, according to The Daily Telegraph.
The proposal has also received the support of Ukip's only MP, Douglas Carswell, and Labour Eurosceptics Kate Hoey and Graham Stringer.
Former minister and member of Conservatives for Britain, Owen Paterson, issued a warning to Cameron on 14 June over the issue.
The Eurosceptic told the BBC's Andrew Marr show: "This is a heartfelt plea to the government: if it [the referendum] is seen to be rigged, if the British people don't think it's fair, then whatever the result it won't be seen to be legitimate and this whole issue will fester further."
But the Tory rebels would need the support of Labour to get their amendment passed as the EU referendum bill goes through its committee stage.
Harriet Harman, the acting Labour leader, tabled her party's own front-bench change to the draft legislation on 12 June.
The amendment would require the government, before a date is set for the referendum, to release a report on what materials the government would publish in the four weeks before the historic vote.
A Labour source told IBTimes UK the condition would enable the government to continue with its normal functions but it would stop Cameron from "pulling a fast one and issuing a [pro-In] leaflet to 40 million people" ahead of the referendum.
The SNP, now the third largest party in the Commons with 56 MPs, also tabled its own amendment to the EU referendum bill on 12 June.
The draft change, put forward by Alex Salmond, would introduce a purdah period of four weeks in the run-up to the historic vote.
But the nationalists, when asked where they are voting on 16 June, did not give much away as a SNP spokesman told IBTimes UK "all will become clear tomorrow".