An epic battle between the government and Labour peers over plans to change the way MPs are elected could be nearing its end in the Lords.
Ministers aim to complete the committee stage of the Parliamentary Voting System and Constituencies Bill later.
But Labour is still resisting plans to cut the number of MPs and redraw constituency boundaries.
The bill, which MPs have approved, must become law by 16 February for a planned referendum to be held on 5 May.
The BBC understands talks are going on behind the scenes between coalition ministers and Labour peers over a possible compromise – with the government under pressure to give more leeway over plans to standardise the size of constituencies.
Wednesday’s debate will be the bill’s 14th Lords committee day – thought to be the most since 1971, when peers spent 18 days on the Industrial Relations Bill.
Under the AV system, voters rank candidates in their constituency in order of preference.
Anyone getting more than 50% of first-preference votes is elected.
If no-one gets 50% of votes, the candidate with the fewest votes is eliminated and their backers’ second choices allocated to those remaining.
This process continues until one candidate has at least 50% of all votes in that round.
Vote reform: Where parties stand Q&A: Alternative Vote referendum
Three further provisional days have been scheduled for the committee stage next week, if no deal can be reached on Wednesday.
If Royal Assent is not received by 16 February, the referendum date could be in doubt – although ministers may seek to force the measure through the Lords by a series of “guillotine” motions.
This would be unprecedented and likely to result in widespread opposition in the House of Lords, which traditionally is self-regulating when it comes to debates.
Another option could be to find a way of reducing the statutory 10 weeks granted to the Electoral Commission to prepare for a referendum, to allow the bill to complete its passage, the BBC understands.
The bill’s report stage and third reading are still to come.
Ministers have accused Labour peers of time-wasting as they seek to separate the referendum on a move to the Alternative Vote (AV), which they support, from the bill’s plans to cut the number of MPs from 650 to 600.
Coalition ministers insist they are part of the same package of reforms.
Peers had already spent more than 98 hours debating the bill by Tuesday morning, including a two-day second reading in November and a 21-hour all-night sitting last week.
Tuesday’s debate lasted eight and a quarter hours, as peers dealt with 10 out of the remaining 50 groups of amendments.
But there were signs that Labour peers’ alleged delaying tactics had finally ended, as one amendment was dealt with in 34 minutes, the next in 11, another in 17, and the last in just four minutes.
Lib Dem Justice minister Lord McNally was, meanwhile, taken ill during the debate in the early hours of Tuesday. He was taken to hospital with what a ministerial colleague said was gastric flu.
The government has suffered two defeats in the Lords over the planned legislation, and ministers will have to decide whether to risk further delay by asking MPs to overturn them when the bill returns to the Commons.
The Parliamentary Voting System and Constituencies Bill would schedule a referendum on bringing in the alternative vote (AV) system for Westminster elections on 5 May.
It also contains provisions for the number of MPs to be cut and for constituency boundaries to be re-drawn so that they are of roughly equal size.
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