The coalition Government faces its first big test in the Commons as MPs return from their summer break when plans for electoral reform could spark a backbench rebellion.
Some Tories are unhappy with plans to hold a referendum to change the Westminster voting rules on the same day as voters in Scotland, Wales and Northern Ireland elect members of their devolved legislatures.
The referendum, set to be held on May 5, 2011, was a key concession for the Liberal Democrats entering into coalition with the Conservatives. It will propose replacing Westminster's current first-past-the-post system with the Alternative Vote (AV).
The plebiscite will be a test of the coalition's strength, with Prime Minister David Cameron set to campaign against a switch to AV while Deputy Prime Minister Nick Clegg will back the change.
But already there are rumblings of discontent among Tory backbenchers. So far, 43 of their number have signed a Commons motion calling for the date of the referendum to be changed because of fears the vote could effectively be skewed.
Bernard Jenkin (Harwich and North Essex), who tabled the motion, said he would be supporting today's second reading of the Parliamentary Voting System and Constituencies Bill but indicated he was unhappy with its timing.
The former Tory frontbencher said: "I will be supporting the second reading but I am not so sure about the timing of the bill. I don't expect more than a handful of colleagues will be voting against this."
Labour will argue that the referendum on AV - which the party backed in its manifesto - should not be combined with measures to reduce the number of MPs from 650 to 600 and to harmonise constituency boundaries, which shadow justice secretary Jack Straw has likened to "gerrymandering".
He will be backed by the SNP who have refused to support the Bill unless the proposed date of the poll is changed so it does not clash with the elections to the devolved administrations.
Writing in the Sunday Telegraph ahead of the vote, Mr Cameron and Mr Clegg said: "We both recognise that there are concerns about the current system. And we agree that the decision is not, in any case for government alone. It should be taken by the people themselves. That is why both our parties support putting this question to a referendum next May."