Britain's centre-right government is facing a potentially devastating crisis triggered by irreconcilable differences over a fundamental reform of the House of Lords, the unelected upper house of parliament, DPA reported.
After the reform was kicked into the long grass by opponents in the Conservative Party of Prime Minister David Cameron last month, Liberal Democrat leader Nick Clegg on Monday threatened to withhold support for another key legislative project.
He accused the Conservatives of a "breach of the coalition contract" over Lords reform and announced that his party, in turn, would not back plans to redraw constituency boundaries - seen as benefiting the Conservatives.
Clegg's publicly-announced decision marks the first open clash over policy between the coalition partners since the government was formed, in May 2010.
Clegg said it was clear that agreement on Lords reform - one of the key goals of his Liberal Democrats party - could not be reached.
Under the plans, the total number of House of Lords' members would have been halved to 450, and 80 per cent of the upper chamber would have had to be elected - rather than being
appointed or taking seats by hereditary privilege.
In a vote in July, more than 90 per cent of Conservative members of parliament (MPs) defied the government on the issue by rejecting a time limit for the bill's passage through parliament.
Cameron, who has said he backs Lords reform, has been unable to control party members
who oppose the plans.
In turn, the Liberal Democrats will now not back related plans to reduce the number of MPs in the Lower House, the House of Commons, from 650 to 600.
Clegg said that while he remained committed to the coalition, it remained an alliance based on "reciprocal agreement" and "mutual respect."
"Clearly I cannot permit a situation where Conservative rebels can pick and choose the parts of the contract they like," he warned.
For the Conservatives, Culture Secretary Jeremy Hunt described Clegg's decision as "disappointing," but added that it would not affect the two parties' commitment to work together.
With parliament currently in summer recess, the issue is expected to return with a vengeance when sittings resume in September, when the traditional party conference season also begins.