Defence Secretary Bob Ainsworth today attacked as "ridiculous" the Liberal Democrats' position over Trident.
Mr Ainsworth claimed the party's policy of saying no to a like-for-like replacement of the nuclear missile system would leave Britain with a "cheap and ineffective" deterrent.
His comments came as the spotlight turned more sharply on Liberal Democrat policies following Nick Clegg's widely praised performance in the first party leaders' debate.
Today's YouGov daily survey for The Sun put the Lib Dems on 30 per cent to Labour's 28 per cent, with the Conservatives in the lead on 33 per cent.
The Tories have launched a sustained attack on the Liberal Democrats' "eccentric" policies in a bid to offset the impact of Mr Clegg's performance in the televised debate.
In an interview with the BBC, Lib Dem defence spokesman Nick Harvey said it was "absurd" to decide now that Britain was "absolutely determined" to be a nuclear power from 2030 - the time he said when the current Trident system will expire.
He hoped instead that "significant headway" could be made before that date on moves towards multilateral disarmament.
The party's policy on Trident rules out a replacement for the submarine-based weapons system, which the Lib Dems say will cost £100 billion. Both Labour and the Tories have pledged to replace it.
Mr Harvey told BBC Radio 4's Today programme: "The existing Trident is there, it is paid for, it is going to work for the next 20 years.
"What we are saying is that it is absurd to decide now that we are absolutely determined to be a nuclear power from 2030 through to 2060."
He added: "I would sincerely hope that in the 20 years between now and the point at which our existing deterrent expires, we would have made significant headway on the world stage with moves towards multilateral disarmament.
But Mr Ainsworth told Today: "You have to plan to replace it if you are going to be able to replace it after 20 years.
"Nick's position appears to be that he wants to have an independent nuclear deterrent but he wants a cheap one.
"A cheap one would be a vulnerable and ineffective one and that is the fact. It is a ridiculous policy."
He added: "Now that the Liberal Democrats are getting the kind of focus that they have, then we will be able to examine some of their policies, like for instance Trident.
"You cannot have an independent nuclear deterrent that is cheap and ineffective, and there are silly policies in other areas as well."