David Cameron has said he is committed to a "full" replacement for Trident nuclear weapons after claims the Lib Dems could force it to be cancelled.
Tory MP Julian Lewis said the Lib Dems were boasting about key decisions being delayed until after the next election.
And he urged the prime minister to guarantee he would not scrap Trident as the price of a coalition in the event of another hung Parliament.
The exchange came at Prime Minister's Questions in the Commons.
BBC deputy political editor James Landale said Mr Lewis was engaged in a "constant tussle" with Mr Cameron over Trident and had been "very disappointed" when the final "main gate" decision, which would see the key contracts signed, was delayed until after 2015 for a "mixture of financial and political reasons".
The Conservatives wanted to avoid a "row" with the Lib Dems before the election but it was significant, said former Labour foreign secretary Jack Straw, that for the "first time he (Cameron) has added the critical adjective 'full' to the replacement for Trident".
There has been speculation that the number of Trident submarines could be cut or continuous sea patrols ended to save cash - or even that a cheaper alternative to the sea-borne system could be commissioned.
In the Commons, Mr Lewis said Mr Cameron had promised Tory MPs at the time the coalition was being formed that the Lib Dems would support a replacement for Trident.
But now that the decision had been delayed, the Liberal Democrats "from their president down have been boasting that this was their achievement" - and he asked Mr Cameron for reassurance that it would not be scrapped as the price of another coalition deal.
Mr Cameron assured him that "the replacement for Trident is going ahead".
"The reason for the delay is that we had a value for money study because we desperately need to save some money in the Ministry of Defence so that we can invest in frontline capability
"In terms of the future, all I can say... is that I am in favour of a full replacement for Trident, for continuous at-sea deterrent, and to make sure we keep our guard up.
"That is Conservative policy. It will remain Conservative policy as long as I am the leader of this party."
Labour MP John Woodcock, whose Barrow-in-Furness constituency builds the Trident submarines, also asked Mr Cameron for reassurance that he would not go back on his word.
Mr Cameron said: "I profoundly believe we should maintain our independent nuclear deterrent. I have looked at all of the alternatives over the years and I am completely convinced that you need a submarine based alternative - a full replacement for Trident in order to guarantee the ultimate insurance policy for this country."
He said he believed there was "all-party support" for his position.
In their election manifesto, the Lib Dems, who have traditionally been in favour of nuclear disarmament, opposed a "like-for-like" replacement for Trident on cost grounds.
It was one of the issues on which they were allowed to continue expressing a different view to the Conservatives in the coalition agreement.
But they were accused of "playing politics" with the issue last year when Lib Dem defence minister Nick Harvey said delaying the decision until after the next election would turn Trident into a political "hot potato" for Labour.
Labour have supported replacing Trident since the 1990s but some of their MPs are thought to be unhappy with the policy.