A decision to assemble Britain's Trident nuclear deterrent in the US to save UK taxpayers £500 million would eventually result in 2000 job losses in Scotland, documents from the National Archive show.

Ministers realised the ultra-sensitive issue for the Thatcher Government of replacing Polaris with a new £7.5 billion Trident system would cause alarm because of the cost, and were thus keen to reduce it wherever possible.

In one memo to the Prime Minister, Cabinet Secretary Sir Robert Armstrong explained how, with Polaris, the missiles were assembled at Coulport from US-made parts and the warheads were then added at the Scottish base.

Mrs Thatcher was told that a decision in favour of "US processing" could not be reversed without delaying the programme.