MPs will not be able to throw out the government unless at least 55% of them vote to do so, under plans agreed by the Conservatives and Lib Dems.
The move is part of plans agreed by the two parties to introduce five-year fixed-term parliaments.
It takes a simple majority of MPs to currently win a confidence vote - and the usual practice is that an election is called if the government loses.
The plans were drawn up during long meetings between Lib Dems and Tories.
The resulting document promises to create a "strong and stable" government, with elections held on fixed dates every five years.
At a press conference, the prime minister said he wanted the coalition to last until the end of the parliament, in May 2015.
The Conservatives currently have 306 out of 649 MPs - a 47% share.
One seat, Thirsk and Malton, is empty, pending a by-election on 27 May, while Sinn Fein's five MPs have not taken the oath of allegiance allowing them to sit in Parliament.
All this means it would be impossible for opponents, even if fully united, to muster the 55% needed to oust the government, unless at least 16 Tories rebel.
Constitutional expert Peter Hennessy, of Queen Mary University of London University, told BBC News: "The tradition is that one [vote] is enough and I wouldn't tinker with that. I would leave that well alone. It looks as if you are priming the pitch, doctoring it a bit. Not good. It's meant to be a different politics, new politics."
Under the deal with the Conservatives, Lib Dem MPs would be expected to vote with the government.
'Power of recall'
There is, however, a compromise aimed at overcoming the parties' differences over nuclear power.
The statement reads: "We have agreed a process that will allow Liberal Democrats to maintain their opposition to nuclear power while permitting the government to bring forward the national planning statement for ratification by Parliament so that new nuclear construction becomes possible."
But there will be "clarity that this will not be regarded as an issue of confidence", it adds. Renewing nuclear power is also backed by Labour, which has 258 MPs.
Among other plans, the coalition government will bring forward legislation for a "power of recall", allowing voters to force a by-election where an MP is "found to have engaged in serious wrongdoing".
This would follow a petition being signed by 10% or more of their constituents.
The parties have also agreed to set up a committee to look at establishing an elected House of Lords.