The newly formed Cameron-Clegg coalition Government in the UK is facing its first threatened backbench rebellion, over a controversial plan to reduce the risk of it being voted out of office before the end of its planned five-year term.

As the new Conservative and Liberal Democrat Cabinet met for the first time today, some senior Tory and Labour MPs voiced concern over a proposal in the coalition agreement that dissolution would need to be approved by 55% or more of MPs.

It would represent a radical shift away from the Commons tradition that a simple majority of one would be enough on a no confidence vote to force an unpopular government to resign.

The 55% threshold means that Mr Cameron could survive at the head of a minority Conservative government even if the Lib Dems pulled out of the coalition deal.

It would need a significant rebellion by disaffected Tories joining force with all the opposition MPs to force him to call another election.

With 306 MPs – likely to be 307 after the delayed election in Thirsk and Malton on May 27 – the Conservatives have just over 47% of MPs. That means the combined weight of all the other parties could not reach the proposed 55% threshold on their own.

Under the agreement thrashed out by the Conservatives and Lib Dems during several days of negotiations, legislation will be introduced to provide for fixed-term five-year parliaments, with the next general election due to be held on the first Thursday of May 2015.

“This legislation will also provide for dissolution if 55% or more of the House votes in favour,” the coalition agreement published yesterday said.

Many MPs question whether the coalition will last that long, despite the promises made by Mr Cameron and his Lib Dem deputy Nick Clegg that it is a long-term arrangement for a full parliamentary term.

Labour MPs are already alarmed by what they see as an attempt to change long-standing Commons rules in the new Government’s favour. They have been joined by several senior Conservatives on the libertarian wing of the party who are lobbying behind-the-scenes for the 55% threshold to be removed before the legislation is introduced in the Commons.

The last time a government fell on a no confidence vote was in March 1979, when the minority Labour administration led by James Callaghan was defeated by 311 votes to 310.

The confidence vote followed the government’s defeat in a referendum on devolution and the collapse of the Lib-Lab pact. The subsequent general election was won by Margaret Thatcher, ushering in 18 years of unbroken Conservative rule.

The most famous confidence vote was in May 1940, the so-called Norway debate, after a British debacle in the early years of the Second World War. The Government’s majority was cut from 231 to just 81 and prime minister Neville Chamberlain resigned, making way for Winston Churchill to lead the nation to eventual victory.