Senior peers warned Nick Clegg last night that he will face a strong rearguard action by them over his plans to turn the House of Lords into an elected chamber.
Today the Deputy Prime Minister will try to revive his constitutional agenda after his defeat in the AV referendum by outlining proposals for 80 per cent of the members of the Lords to be elected by proportional representation.
The size of the second chamber would fall from about 750 to 300 under Mr Clegg's draft Bill. Its functions and powers as a revising chamber would remain the same.
Mr Clegg wants legislation on the statute book before the next election but faces opposition from many peers and only lukewarm support from some Conservatives. Forty peers from all parties fired off a letter to MPs last night warning that the "supremacy and authority" of the Commons would "inevitably be challenged" by an elected Lords.
The backers, including the former Liberal Party leader Lord (David) Steel and the former Commons Speaker Baroness (Betty) Boothroyd, said the existing all-appointed Lords has "very real merit" because it can reflect the gender and ethnicity balance of the country. An elected second chamber would take away its "vast reservoir of talent and expertise".
Mr Clegg also faces criticism from Labour for not going far enough. The Opposition will accuse him of watering down plans for a 100 per cent elected Lords, which appeared in the Labour and Liberal Democrat manifestoes last year, to appease the Conservatives.
"A fully elected Lord should be the starting point," said a Labour source. "He shouldn't be making concessions to the Tories yet."
However, Clegg aides denied he had abandoned hopes of a 100 per cent elected chamber. The option will be kept alive in a White Paper published alongside the draft Bill.
Liberal Democrat sources said Labour should rally behind the proposals rather than pick holes in them. They said Lords reform was a big test for Ed Miliband and urged him to show leadership on an issue where there is common ground between Labour and the Liberal Democrats.