Search: Voting reform referendum
MPs are due to vote on proposals to hold a referendum on abandoning the first-past-the-post system for general elections.
Under electoral reform plans championed by Gordon Brown, the country would be asked to choose whether to switch to the alternative vote (AV) method.
The Prime Minister hailed the change as part of a "new politics" which could restore public trust in Westminster in the wake of the expenses scandal.
But the Tories will oppose the move which Opposition leader David Cameron dismissed on Monday as a "fiddle", accusing the PM of "a cynical attempt to save his own skin".
Amendments tabled by Justice Secretary Jack Straw to the Constitutional Reform and Governance Bill would require a referendum to be held by the end of October 2011.
It would offer a straight choice between the existing system and AV - under which voters would rank candidates in order of preference.
The Liberal Democrats will seek to change the question to offer the choice of a more proportional system - the single transferable vote - instead of AV.
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They also want to bring forward the last date of the referendum to May next year and close what they say is a loophole that would allow an incoming Conservative administration to abandon the vote without the need for fresh primary legislation.
Campaigners for voting system change welcomed the AV proposal as a "small step" towards making the electoral system more representative.
Mr Cameron called at the weekend for the "crazy" vote to be dropped and for MPs to delay their half-term holiday by a day to debate Parliamentary reforms proposed by a committee chaired by Labour backbencher Dr Tony Wright to strengthen scrutiny of the government.
