Former British Prime Minister Sir John Major was warned the nation's Polaris nuclear submarines had an "inadequate and vulnerable" communications system.

The warning came in a briefing to Major, classified as secret, in 1990 by his private secretary Charles Powell ahead of a meeting with the then defence secretary Tom King.

Britain's nuclear submarines have been stationed at the Clyde Naval Base, Faslane since 1968.

The note to the Prime Minister was among 2000 files released by the National Archives on Thursday.

King met Major in Downing Street to discuss the issue on August 2, 1990.

In the note to the Prime Minister, Powell said: "We shall need a decision soon on a new system for communication with our Polaris, and later Trident, submarines.

"The present system is inadequate and vulnerable, but will be very expensive to replace. Further ahead we have to decide on [the] acquisition [of] a tactical air-to-surface missile to replace our free-fall nuclear bombs.

"In the past we have always bought American: this time there is some pressure to buy French for political reasons. But we do not want to end up with a 'diplomatic' weapon."