Nick Clegg has accused Defence Secretary Philip Hammond of "jumping the gun" on Trident as coalition tensions grew over the future generation of British nuclear-armed submarines.
The Deputy Prime Minister dismissed suggestions that an additional £350 million of funding announced for designing a replacement deterrent "made clear" the Government's commitment to maintaining the system.
Mr Clegg insisted the coalition agreement will not be "undermined or contradicted" and the Liberal Democrats' review into alternatives will continue.
He said: "Having seen the papers this morning, I think some people are jumping the gun on this Trident decision. The coalition agreement is crystal clear - it will not be changed, it will not be undermined, it will not be contradicted.
"The final decision on Trident replacement will not be taken until 2016, however much other people may not like it that way."
Mr Hammond is making the announcement on a visit to the home of the UK's nuclear deterrent at Faslane on the River Clyde in Scotland. The funding was already earmarked as part of the design stage but the "maingate" decision is not due to be taken until 2016.
The Liberal Democrats are understood to be unhappy that the latest stage is being used to indicate that a replacement will go ahead regardless of any recommendations that come out of a review due to report back at the end of the year.
Mr Clegg added: "The idea of a like-for-like entirely unchanged replacement of Trident is basically saying we will spend billions and billions and billions of pounds on a nuclear missile system designed with the sole strategic purpose of flattening Moscow at the press of a button."
The funding will sustain 1,200 UK jobs and follows the initial £350 million of design work announced earlier this year, the Ministry of Defence said.
Downing Street said it was necessary to commit funding now to the design and development stage of the replacement submarine programme. "As the Defence Secretary has made very clear, we are progressing on the design and development. The decision on construction will not be taken until 2016," a Number 10 spokeswoman said.