Nick Clegg's plan for a May referendum on changing the electoral system is being threatened by a power-struggle in the House of Lords.
The row could mean the vote cannot be held on the Deputy Prime Minister's chosen date of May 5 and could be delayed for several months.
Peers are battling over the Parliamentary Voting System and Constituencies Bill, which would allow the referendum on replacing the first-past-the-post system at Westminster with the Alternative Vote method.
Labour peers are threatening to hold the legislation up in the House of Lords, meaning it would miss a deadline for being passed into law in time for a May referendum.
Failing to hold the vote on Mr Clegg's chosen date would be a political blow for the Deputy Prime Minister, who was last night braced for a Lib Dem defeat in the Oldham East and Saddleworth by-election.
Mr Clegg has summoned Lib Dem ministers to an 8am meeting on Friday to discuss "the implications of Oldham" for their party.
Labour peers have so far dragged out the committee stage of the bill – which considers its clauses in detail – to eight days of debate, and there is still much ground to cover.
In response to what ministers see as Labour delaying tactics, Government whips have threatened to make the Lords sit through the night next week to test peers' willingness to keep the debate going.
As well as allowing the referendum, the legislation also enacts Conservative plans to redraw constituency boundaries and shrink the House of Commons.
Labour has strongly opposed the redrawing plan, fearing that it would effectively abolish many safe Labour seats.
Party sources said the delaying tactics in the Lords are aimed at forcing the Coalition to split the legislation into two separate bills, one for the referendum and one for the redrawing plan.
Lord Falconer, the former Labour lord chancellor, defended Labour's tactics, saying it was right to take time debating the bill.
He said: "This Government is trying to ram this through in an arrogant way. We will ensure proper scrutiny. We will do what the House of Lords is there to do."
Downing Street insisted that the bill will be passed on time, even if it means clearing the House of Lords schedule to allow several more days of debate.
Upping the stakes, Government whips have removed time limits on Monday's sitting, meaning the Lords could sit through the night if Labour peers try to prolong the debate.
A No 10 spokesman said the Government is "confident" the bill will pass in time. He said: "The bill remains as it is. It is our intention to take it through. We are confident that the legislation will be in place to allow the referendum to take place on May 5."