Senior West MPs are among the Tory rebels threatening to join forces with Labour to inflict a first damaging Commons defeat on the coalition Government.

They are campaigning against the decision to hold a referendum on changing Britain's voting system next May, on the same day as local elections.

The legislation allowing the referendum will also reduce the number of MPs, which Labour opposes – triggering a furious attack by Prime Minister David Cameron yesterday.

So far 45 Tory MPs – and two from Labour – have signed a Commons motion protesting at the decision to hold the referendum on May 5.

They include former frontbenchers Geoffrey Clifton-Brown (The Cotswolds) and Laurence Robertson (Tewkesbury) and senior backbenchers Ian Liddell-Grainger (Bridgwater & West Somerset) and James Gray (North Wiltshire).

New MPs Jack Lopresti (Filton & Bradley Stoke) and Jacob Rees-Mogg (North East Somerset) have also signed, along with former Cabinet Ministers such as Sir Malcolm Rifkind, John Redwood and Peter Lilley.

The MPs are worried that holding the poll on whether to bring in the Alternative Vote (AV) system on the day of other elections will favour the "Yes" campaign.

Mr Lopresti said his support for the traditional first past the post voting system was well known.

He said: "We are honour bound to have a referendum but we have serious concerns about the timing of it. I don't think there is any conflict between the party line and saying hold the referendum at a different time."

Mr Gray asked why the referendum would not offer the single transferable vote system, which many people think is fairer. It would just include AV, which Liberal Democrat leader Nick Clegg had previously denounced as "a pathetic excuse for a voting system".

Mr Rees-Mogg said: "I'm concerned about all of the constitutional changes that are being proposed, they are being brought forward at a gallop. They are a result of the coalition talks and were not part of anyone's manifesto.

"I do think MPs are there to independently-mindedly represent constituents. I think AV is nonsense, it is a really bad system that tends to exaggerate results.

"It makes election trends more extreme and gives unsuccessful parties a second bite of the cherry."

The PM yesterday accused Labour of "a descent into complete opportunism" after hearing the party will vote against the legislation.

He urged Labour to resist "the temptation to jump on the bandwagon" and claimed to be "confident" of getting the legislation through.

And Lib Dem deputy leader Simon Hughes said it was staggering hypocrisy, as most of Labour's leadership contenders back AV.