Midland MPs have joined a rebellion over House of Lords reform which could hand David Cameron his first major Commons defeat and threaten the future of the Coalition government.

A number of Midland Conservatives have signed a letter warning they are prepared to defy the Prime Minister over plans to create an elected second Chamber.

It means the Government could face defeat in a crunch vote tonight .

But Lords Reform is a key issue for the Liberal Democrats – who may retaliate if the proposals are blocked, by refusing to support Conservative proposals to cut the number of MPs in the Commons from 650 to 600.

Around 70 Conservative MPs have backed a letter warning that House of Lords reforms threaten to “pile a constitutional crisis on top of an economic crisis”.

It was organised by MP Jesse Norman (Con Hereford). Other signatories include Chris Kelly (Con Dudley South), Chris Pincher (Con Tamworth), Robin Walker (Con Worcester), Nadhim Zahawi (Con Stratford), Bill Cash (Con Stone), Aidan Burley (Con Cannock) and Dan Byles (Con North Warwickshire and Bedworth).

Birmingham Conservative Andrew Mitchell (Con Sutton Coldfield), a Cabinet Minister, will support the Government in tonight’s vote, which will limit the amount of time MPs can spend discussing the issue to 10 days.

Without a strict time limit in place it will be extremely difficult for the Government to get the legislation through the Commons because opponents will be able to cause delays by making lengthy speeches.