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The Liberal Democrat policy on Trident has been called "ridiculous" by Defence Secretary Bob Ainsworth.Labour and the Conservatives both want to renew the UK nuclear weapons system when Trident expires in the 2020s.
The Lib Dems "rule out like-for-like" replacement, instead saying they will seek "alternatives" and support efforts at multilateral nuclear disarmament.
Mr Ainsworth claimed the Lib Dem policy would leave Britain with a "cheap and ineffective" deterrent.
But Lib Dem defence spokesman Nick Harvey told BBC Radio 4's Today he hoped "significant headway" would be made in the next 20 years towards multilateral disarmament.
He said: "It is absurd to decide now that we are absolutely determined to be a nuclear power from 2030 through to 2060."
'Another view'
He said in turn it was wrong to commit billions of pounds to Trident replacement now. The Liberal Democrats say like-for-like replacement would cost £100bn and have said they would consider cheaper alternatives.
The government has put the bill for Trident renewal at between £15bn and £20bn.
"I would sincerely hope that in the 20 years between now and the point at which our existing deterrent expires that we would have made significant headway on the world stage with moves towards multilateral disarmament," Mr Harvey said.
But Mr Ainsworth said plans had to be made now to replace Trident if the new system was to be ready in time.
He said: "Nick's position appears to be that he wants to continue to have an independent nuclear deterrent but he wants a cheap one.
"And a cheap one would be a vulnerable and ineffective one and that is the fact. It is a ridiculous policy."
The SNP's Angus Robertson told the programme the three main UK parties were prepared to spend "scores of billions on weapons of mass destruction with no other mainstream party giving another view".
"That is what the SNP is doing in Scotland, " he said. The SNP would scrap Trident and permanently rule out nuclear weapons in Scotland.This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.