The threat of a voting reform referendum on May 5 being scuppered by political deadlock in the House of Lords was eased today after Tory and Labour peers struck a deal.
But critics warned the battle was not over and that further agreements would be required if the vital deadline for approving the legislation was not to be missed.
News of the compromise emerged amid reports David Cameron was willing to provoke a constitutional crisis by imposing a "guillotine" curtailing debate in the upper chamber.
It is believed to centre on changes put forward by the convenor of the crossbench peers, Baroness D'Souza.
The scheduled date for a public vote on switching to the Alternative Vote system was put in peril by protracted debate in the Lords, so far spanning more than 80 hours over 14 days.
Ministers have accused Labour of filibustering to delay the legislation, which must become law by February 16 for the referendum to be able to go ahead on the planned date.
Labour peers oppose measures contained in the same legislation to reduce the House of Commons from 650 to 600 MPs - which the party says are designed to disadvantage it.
Lords leader Lord Strathclyde signalled a breakthrough tonight, promising a "package of concessions" during a statement to peers.
They will bring the prolonged committee stage of the Parliamentary Voting System and Constituencies Bill to an end on Wednesday.
But former lord chancellor Lord Falconer, who described the possibility of a guillotine as "an abomination", said the changes were not yet sufficient to ensure progress.
"We have agreed through the usual channels the timetable for the completion of (the committee stage of) this Bill at the close of business this Wednesday," he told peers.
"Focusing only on the key issues on report, and employing the economy and focus your lordships will expect at report, and later at third reading, the further timetable depends on further agreement on substantive issues between the parties."
Constitutional affairs minister Mark Harper indicated earlier that changes proposed by Baroness D'Souza were among those where the Government could compromise.
"The two things that D'Souza's been looking at are a post-legislative review of the numbers - what happens when you reduce to 600 MPs, what sort of effect does it have? We're very happy to look at that. We've already been having some talks with some of her colleagues," he said.
"The other one is about can we reintroduce some oral element into the consultation process, for evidence. We're very happy to look at that as long as...you get the review done in time."Reuse content