LONDON — Britain's government Monday dropped plans for a mainly elected upper house of parliament, sparking one of the biggest crises in Prime Minister David Cameron's coalition formed two years ago. Deputy Prime Minister Nick Clegg said Cameron's Conservative party had broken the contract with his Liberal Democrats by failing to back their flagship House of Lords reforms. In a retaliatory move which will put severe strain on the partnership, Clegg announced that his centrist party will now drop its support for key Conservative plans to alter the boundaries of parliamentary constituencies. “The Conservative party is not honoring the commitment to Lords reform and as a result part of our contract has now been broken," Clegg told a press conference. “So I have told the prime minister that when, in due course, parliament votes on boundary changes for the 2015 election, I will be instructing my party to oppose them." Conservatives have long claimed that the current boundaries give them a disadvantage at the polls, and their junior partners' move to block the changes is one of the most serious blows to the coalition since it formed in May 2010. A rift between the parties has widened in recent months over several issues, which include Lords reform but also Britain's relationship with the European Union. Clegg had championed plans to remove the last hereditary peers from the House of Lords — an institution that dates back to the 14th century — and reduce its 800-strong membership to around 450. “An unelected House of Lords flies in the face of democratic principles and public opinion," he told journalists as he announced that the reforms had been dropped. “It makes a mockery of our claim to be the mother of all democracies." The majority of the House of Lords' members are appointed by political parties, while the rest include 26 archbishops and bishops and 92 hereditary peers. Clegg has previously argued that Britain is the only country in the world apart from Lesotho to have a completely unelected upper parliamentary chamber. Both the Lib Dems and the Conservatives promised changes to the Lords at the last general election in May 2010, along with the opposition Labor party, but it is the Lib Dems who have been the strongest proponents. Critics argue that elected membership of the upper house, which scrutinizes legislation before it passes, would undermine the supremacy of the House of Commons. Last month the coalition was forced to drop a key parliamentary vote designed to speed up the Lords reforms after around 100 Conservative rebels threatened to vote against it. On Monday, Clegg accused Conservatives of trying to “pick and choose" policies agreed with the Lib Dems when they formed the coalition. — Reuters