MPs will be given the chance to veto replacing the Trident nuclear weapons system, says Commons Leader Jack Straw.
MPs are to be a given a say on replacing Trident
Tony Blair has previously promised a full debate on the issue but stopped short of pledging a vote.
But Mr Straw said it was "important" MPs had a say and said they would get a substantive yes or no vote - meaning they could in theory reject the plan.
It would be "inconceivable" ministers could press ahead with renewing Trident if MPs rejected it, Mr Straw argued.
Questioned by reporters, Mr Straw declined to say when the vote would take place.
But he said he was confident there was majority backing on the Labour benches for renewing Trident.
The Conservatives are also likely to back such a move - meaning the government would win any vote.
Shadow defence secretary Liam Fox tried to reassure US defence chiefs of his commitment to replacing Trident in a visit to Washington earlier this year.
A timetable for renewing the missile system is to be drawn up by the end of the year.
Earlier in the Commons, Mr Straw told MPs: "The position of this was set up by the prime minister two weeks ago when he did point out that we were the first government to give the House a vote over decisions to go to war.
"Of course we should involve the House fully in a decision as important as the renewal of our nuclear deterrent and in practical terms it is inevitable that there will therefore be a chance for the House to express its view on this important matter in a vote."
Mr Straw's announcement was welcomed by the Campaign for Nuclear Disarmament (CND) - but it called for a full public debate on the issue before MPs vote on it - as promised last year by then defence secretary John Reid.
CND chairman Kate Hudson said there had been great demand for a full vote across Labour and other parties.
"We would now like to see the commitment to the publication of a Green Paper outlining all options, including non-replacement, prior that vote taking place," she said.