LONDON (Reuters) - Prime Minister David Cameron faces one of the biggest rebellions of his premiership after more than a fifth of his own party threatened on Monday to vote against plans to reform parliament's upper chamber, the House of Lords.
The Conservative Party revolt also threatens to take a sledgehammer to Cameron's coalition government with the Lib Dems, whose push to make the Lords an elected rather than an appointed chamber is a key condition of the coalition deal.
At least 70 Conservatives signed a letter made public on Monday expressing "serious concern" over the proposals, which they say threaten to pile a "constitutional crisis on top of an economic crisis" as Britain tries to revive its ailing economy.
"I'm voting against the programme motion. I'm completely opposed to Lords reform. This bill is simply a means for the Liberal Democrats to blackmail the Conservative Party," Conservative MP Nadine Dorries told Reuters, adding that more than 100 other Conservatives shared her view.
The Lib Dems argue that an appointed upper chamber is undemocratic and that its aristocratic and privileged membership, resplendent in their scarlet robes fringed with ermine fur, is an anachronism.
"Right now we are one of only two countries in the world, the other being Lesotho, with an upper parliamentary chamber which is totally unelected and instead selects its members by birthright and patronage," Lib Dem leader Nick Clegg said.
The Lords consists of more than 800 members, who review laws and scrutinise the work of the government. The queen appoints members on advice from the prime minister, although some inherit the role and some seats are reserved for members of the clergy.
While many seats used to be passed down with land rights, that was overhauled in a Lords reform in 1999, under a centre-left Labour government, leaving just short of 100 seats held by such hereditary peers elected by their colleagues.
The government wants to cut membership of Lords to 450 by 2025, and make 80 percent of the chamber's seats elected for non-renewable 15-year terms, with the rest appointed by an independent committee on the basis of particular expertise.
Conservative rebels argue that elected Lords would be more partisan and undermine the primacy of parliament's lower chamber, creating legislative gridlock. They also fear an elected chamber would lack diversity and specialist expertise.
Parliament is due to debate the plan on Monday and Tuesday and then vote on the scheduling of the bill's passage through parliament.
Rebels from the Conservative Party - a party which has traditionally drawn MPs from more privileged backgrounds -and the now opposition Labour party want to delay the bill to allow time for debate and amendments.
That could push the legislation into the next parliament after a 2015 election, effectively killing the bill.
Unlike Nadine Dorries, who is a frequent and vocal critic of government policy, other less rebellious Conservative lawmakers are also throwing their weight behind efforts to delay and amend the reforms, a dangerous development for Cameron.
"I am so appalled at the constitutional catastrophe that is the new House of Lords reform bill," Conservative lawmaker Nicholas Soames, who has only once voted against his own party in his 29-year career, wrote in the Daily Telegraph newspaper.
"This bill must be defeated at all costs."
The Lib Dems, who are bumping along the bottom of opinion polls, are desperate to claim Lords reform as a big win.
"All of us here believe in democracy. We believe that the people who make the laws should be chosen by the people subject to those laws," Clegg, also deputy prime minister, said at the start of the debate.
Cameron's spokesman said Conservative MPs had been "whipped" - urged to back the government line - appropriately.
"No one should be in any doubt on his position on Lords reform. He is committed to the reforms ..." the spokesman said.
A defeat by his own party would weaken Cameron as Conservative leader and shake the coalition foundations.
"It will further undermine the credibility of the coalition and undoubtedly Cameron," Nottingham University politics professor Steven Fielding said.
Additional reporting by Michael Holden; Editing by Alison Williams