Nick Clegg has called for the Trident nuclear deterrent to be scrapped, saying it is too expensive and no longer meets the UK's defence needs.
The Lib Dem leader is the most senior politician to say Trident should not be renewed when it expires in 2024.
The UK still needed a deterrent, he told the BBC, but a "like for like" replacement was out of the question.
Labour and the Conservatives back Trident renewal, estimated to cost £20bn, but many Labour MPs oppose it.
When Tony Blair agreed to renew the submarine-based missile system in 2007, 97 Labour MPs opposed the move.
Although long sceptical about the cost of replacing Trident, the Lib Dems had, up to now, adopted a wait-and-see policy arguing that decision on renewal could be postponed.
Nick Clegg says politicians have to ask big questions about what the UK can afford
In the leadership contest of 2007, Mr Clegg clashed with his rival Chris Huhne - a firm opponent of Trident - over the subject.
Mr Clegg told the BBC he had "changed his mind" over the issue and he believed that, 20 years after the end of the Cold War, Trident was now clearly outmoded.
He now says a decision cannot be delayed any longer, given the desperate state of the UK's public finances.
"We have to be realistic and candid about what we can and can't afford as a nation," he said.
"If we don't move with the times, if we don't accept circumstances have changed, if we don't accept our capacity to pay for everything we're paying for at the moment is going to be more limited in the future then I think we're not being candid with the British people, we're not being candid with ourselves."
While hoping that the UK will eventually be able to scrap its nuclear weapons as part of a multilateral agreement, he has asked former Lib Dem leader Sir Menzies Campbell to look at cheaper, alternatives to Trident in the meantime.
Not since the 1980s has an argument about nuclear weapons divided the major political parties
Gordon Brown says Trident is necessary to protect the UK in a volatile world where nuclear proliferation is a major threat to international stability.
The Tories have long supported Trident renewal but are currently reviewing all their spending commitments and say all programmes will be considered in a strategic defence review if they win the next election.
The BBC's Political Editor Nick Robinson said the nuclear issue was now back on the frontline of British politics and would be a factor in the general election for the first time in decades.
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