Labour is drawing up plans to sabotage the Bill proposing a referendum on the voting system during its passage through the House of Lords.
Senior Labour figures believe they can delay the public vote on whether to ditch the first-past-the-post system due to take place next May by using stalling tactics when the Bill is debated in the second chamber later this year.
The Government wants to get the measure on the statute book by the end of the year, but Labour and crossbench peers are likely to demand a longer Lords debate running well into the new year. That could make it difficult for preparations to be completed in time for a May referendum on bringing in the alternative vote (AV) for general elections.
Peers take a close interest in constitutional affairs and ministers cannot use a "guillotine" motion to cut short debate in the Upper House, as they do in the Commons.
A delayed vote would be a setback for Nick Clegg, the Deputy Prime Minister, who believes there would be a higher turnout – and better chance of a "yes" vote – if the referendum takes place on the same day as elections to the Scottish Parliament, Welsh Assembly and English councils next May.
One Labour source said: "By delaying it, we would mess up the timetable. It could even bring the Coalition down if Clegg can't deliver the referendum for his party."
Labour MPs will also join forces with Tory backbenchers who oppose AV by backing their calls for a delay during the later stages of the Bill's passage through the Commons.
Despite strong criticism from some Tory MPs, the Bill was given a second reading last night by 328 votes to 269, a government majority of 59. A Labour attempt to derail the measure was defeated by 347 votes to 254.
Mr Clegg defended the proposed timetable, saying that holding the referendum on the same day as elections affecting 84 per cent of the public would save taxpayers £30m. The Bill also proposes a cut in the number of MPs from 650 to 600 and for constituencies to have roughly the same number of voters – a move which could cost Labour 20 seats.
Mr Clegg rejected Labour charges of "gerrymandering," insisting that the Boundary Commission, not the Government, would redraw the map of constituencies.
Jack Straw, the shadow Justice Secretary, said Labour would support the Bill if it were split into two. Although Labour backed AV, it could not support a measure which included "the worst kind of political skulduggery". He condemned a proposal to scrap local public inquiries so that the new boundaries could be rushed in for the next general election.
The Tory MP Mark Field said: "It should never be the Conservative way to tamper with the voting system or electoral boundaries for narrow party political advantage. The timelessness of our nation's constitutional arrangements is too important for that. Yet the current proposals for AV and the reduction in number of parliamentary constituencies are being promoted by party managers as an expedient way to prevent our principal political opponents from recapturing office."
David Davis, a former shadow home secretary and a leading opponent of the Bill, said the Government should be open about the "party advantage" implicit in the plans. He told MPs: "I think the people might respect us more if we admitted some of the real reasons for what we are doing. Of course there is party advantage implicit in what we are talking about."