The referendum is due to take place on 5 May. Government plans for a referendum in May on changing the voting system could be at risk if peers vote later to refer them to a Lords committee.
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 them.
Electoral reform was a key demand of the Liberal Democrats before they agreed to join the coalition with the Conservatives.
Ministers are determined to push through their planned electoral reforms quickly, with a referendum on the alternative vote system pencilled in for 5 May and planned boundary changes due to be completed by 2013.
Both dates could now be in jeopardy because of claims the Parliamentary Voting System and Constituencies Bill is technically flawed and is “hybrid legislation”, which 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 a parliamentary select committee local appeals process.
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, our correspondent adds.
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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