Government plans for a referendum in May on changing the voting system could be at risk if peers vote later to refer them to legislative experts.
Peers, led by ex-Lord Chancellor Lord Falconer, claim the bill covering the move has not been properly drawn up.
The bill also paves the way for a reduction in the number of MPs from 650 to 600 and the re-drawing of boundaries to create similar-sized constituencies.
Labour says the reforms are designed to rig the electoral system against it.
Electoral reform was a key demand of the Liberal Democrats before they agreed to join the coalition with the Conservatives.
'Detrimental'
Ministers are determined to push their plans through Parliament quickly, with a referendum on the alternative vote system pencilled in for 5 May and planned boundary changes due to be completed by 2013.
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Analysis
Norman Smith
Chief political correspondent, BBC Radio 4
Rarely can a key government bill have been so little loved.
In the Commons, Tory MPs voted for it reluctantly and only because they'd been ordered to by David Cameron for the good of the coalition.
Labour is hopelessly split over the idea of an AV referendum - but view the redrawing of constituency boundaries as a shameless attempt to rig the electoral system because they would lose up to thirty seats.
And even the Lib Dems are not hugely enthusiastic - as they view AV as a second-best option to a fully proportional electoral system.
Pretty much all MPs are a bit queasy about the idea their numbers should be culled by 50, fearing that they could be among those destined for the chop.
Such a lack of goodwill towards the bill in the Commons, may only encourage peers to put the brakes on the bill.
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Both dates could now be in jeopardy because of claims the Parliamentary Voting System and Constituencies Bill is technically flawed and is "hybrid legislation", meaning it makes reference to local issues as well as dealing with the UK-wide system.
Lord Falconer and his colleagues cite references in the bill to keeping the Western Isles and Orkney and Shetland as single constituencies, while other distinct parts of the UK - such as the Isle of Wight - are not mentioned.
They say hybrid bills should be referred to a House of Lords committee so interested parties can lodge appeals.
Labour is expected to table a motion later to refer the bill to parliamentary officials known as "the Examiners". If they rule it is a hybrid bill it will be referred to a Lords select committee which could hear petitions against the proposed law change.
Such a move would severely delay the legislation - by months - and throw the coalition's plans for electoral reform into turmoil, BBC political correspondent Norman Smith says.
Ministers are already facing a daunting timetable since they have only until 24 February to get the referendum legislation on the statute book in order to comply with an Electoral Commission ruling.
If the Labour motion goes through, the government may be forced to split the legislation into two bills in order to save the referendum.
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WHAT IS ALTERNATIVE VOTE?
Instead of marking the ballot paper with an X for one candidate, the voter can rank all the candidates in order of preference.
If a candidate receives a majority of first-preference votes then they are elected.
If no candidate gains a majority of first preferences, then the second-preference votes of the candidate who finished last on the first count are redistributed.This process is repeated until someone gets over 50%.
Representatives are still elected for single-member constituencies.
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Lord Falconer told BBC Radio 4's Today programme: "Reducing the number of MPs by 50, on no real basis, will have a detrimental effect on our politics...
"We would have a weaker House of Commons, rather than a stronger one. We need a stronger House of Commons to hold the government to account."
He added: "I'm not trying to scupper the bill at all. There's nothing to stop the coalition, if it wants the reform [of the size of the Commons], to take it into a separate bill."
But Lib Dem Justice Minister Lord McNally said: "If the House of Lords falls for his elephant trap, then their lordships are not as smart as I think they are."
He added: "We have been through a process of devolution in this country without reducing the overall number of MPs... We want to make the case for fair votes in fair-sized constituencies."
http://www.bbc.co.uk/news/uk-politics-11754656Didn't know all that, half expecting it to be fudged now...