The Ministry of Defence has admitted “further time” is now needed before a crunch decision can be taken on the design of the submarines needed to carry the new nuclear missiles. The decision was originally meant to be made last year.

One official statement released last week suggested the earliest the decision could now be taken was July, which would take it beyond the forthcoming general election. But this timetable was later disowned by the MoD in an apparent attempt to save face, without giving an alternative date.

The main reason for the delay is suspected to be difficulties in agreeing the type of nuclear reactors that will power the submarines. According to one senior official, this was “the most tricky issue we have to deal with”.

The revelation has prompted fierce attacks from opposition politicians and anti-nuclear groups. “The Government’s plans for Trident replacement are unravelling,” said MP Nick Harvey, the LibDems’ defence spokesman.

The weapons system should be up for negotiation at an international nuclear conference this summer, Mr Harvey argued. Even if Britain still needed a nuclear “deterrent” after that, it could be smaller and cheaper than the current Trident system.

The UK Government, backed by a majority of the Westminster Parliament, took the decision in principle three years to replace Trident submarines. But before this translates into reality, two other major decisions have to be made, known in MoD jargon as ‘initial gate’ and ‘main gate’.

‘Initial gate’, which involves approving the major designs for the submarines, was originally due to be taken last September, then postponed until December. It is now clear, however, that it has been postponed again, this time without any fixed deadline.

An official summary of a meeting of the MoD’s Defence Board, one of its top decision-making bodies, on November 26 last year was released on request to the Sunday Herald. The summary said the board “took stock of progress on the successor submarine programme, and the challenges that remained before it could be initially considered by the Investment Approvals Board next July”.

This suggests that the many hundreds of millions of pounds of taxpayers’ money involved in the ‘Initial Gate’ decision will not be committed until at least July. But when this was put to the MoD, a spokesman claimed the official summary was mistaken.

The language “was based on an early draft of the Defence Board minutes that did not fully describe the position,” he said. “This will be amended when the final minutes are published.”

The MoD spokesman insisted “no final decisions” had yet been made on the timing of the ‘Initial Gate’ decision. Although it had been planned for last autumn “further time has been required to ensure we take decisions based on the most robust information,” he added.

The Scottish Campaign for Nuclear Disarmament welcomed the delay and said it would ensure Trident was an election issue. Its chairman, Alan Mackinnon, said: “Every candidate who stands for election should make it clear whether they are going to cut Trident or vital public services.”