A government review will provide "credible" alternatives to a direct replacement of Trident, Treasury Chief Secretary Danny Alexander has said.
The Lib Dem cabinet minister said a new continuous sea-based nuclear deterrent was "not financially realistic".
Mr Alexander took charge of the review after a cabinet reshuffle in the autumn.
In an interview with the Guardian, he said a like-for-like replacement for Trident was not necessary.
'No magic pot'
Mr Alexander said: "Given all the financial pressures across the whole of the public sector, all the things the government has to do and wants to pay for, and all the pressures in different areas, I just think the idea that somehow, out of thin air, we can carve a multibillion pocket to pay for this, that is not financially realistic.
"We are in a position where the costs of the successor have to be paid for from within the Ministry of Defence budget. There is no magic pot of money that is going to be created out of thin air to go on top of that.
"As a government, we have been very clear about that. Certainly myself and the chancellor."
He added: "I would expect we will be able to set out serious, credible arguments and potential alternatives."
The Liberal Democrats secured the review as part of the coalition deal with the Conservatives in May 2010. It is due to report in June.
The government is spending £1.4bn on initial design work for Trident replacement submarines but the final decision about whether to go ahead is not due until after the next general election.