Coalition tensions over the future of Trident had flared up as Nick Clegg warned Defence Secretary Philip Hammond over "jumping the gun" on the future generation of British nuclear-armed submarines.
The Deputy Prime Minister insisted that the final decision on replacing the deterrent would not be made until 2016, "however much other people may not like it that way", as £350 million in funding was announced for designing a new system.
Liberal Democrats are understood to be unhappy that the latest wave of cash is being used to indicate that a replacement will go ahead regardless of any recommendations that come out of its official investigation into other options.
The Trident Alternatives Review, headed by chief secretary to the Treasury Danny Alexander, is being carried out by the Cabinet Office and will report back in the next few months.
A Ministry of Defence release said the investment "makes clear the Government's firm commitment to maintaining continuous at-sea deterrence for future decades". Mr Clegg 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.
"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."
Despite the MoD describing the money as "additional" funding, the Lib Dems insisted it was not new, and defence officials later confirmed it was the latest wave of the £3 billion announced last year for the design stage. Mr Hammond announced the cash, which it is said will sustain 1,200 UK jobs, during a visit to the home of the UK's nuclear deterrent at Faslane on the River Clyde in Scotland.
The SNP and the Campaign for Nuclear Disarmament insist that Trident supports only 520 jobs, but Mr Hammond said this "relates simply to one part of the facility here for weapons handling". Moving Astute and Trafalgar-class attack submarines there will create 1,500 "essentially new jobs" which will move to Scotland from the facilities where they are based, he said.
Scottish Deputy First Minister Nicola Sturgeon attacked the UK Government for "dumping" weapons of mass destruction on Scotland, and said there was cross-party support for scrapping the renewal of Trident in the Scottish Parliament.