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A nuclear submarine crash off the British coast which was kept secret for more than 40 years could have triggered World War Three.

CIA documents have revealed the full extent of the crash between a Soviet and US vessel close to Holy Loch in Argyll, Scotland in 1974 - just as relations between the two nations were at an all-time low.

The 1974 smash, which occurred as the Soviet sub trailed its US counterpart, was hushed up until recently, when millions of secret documents were published online.

The SSBN James Madison, a Poseidon submarine carrying 160 nuclear warheads , had just left a US Navy base at Holy Loch before the collision with the Soviet sub.

US Secretary of State Henry Kissinger was informed of the incident in a 'Secret eyes only' message from the Pentagon on November 3, 1974.

(Image: National Geographic)

The note claimed the Soviet vessel was trying to tail the US ship, and said: 'Both submarines surfaced and the Soviet boat subsequently submerged again. There is no report yet of the extent of damage. Will keep you posted.'

But Hans Kristensen, a nuclear weapons expert, told the Times the incident could easily have escalated "if the crew on one of the submarines had misinterpreted the collision as an attack and decided to defend itself and sink the other submarine."

The incident came to light soon after a military expert warned a faulty Trident missile fired by a Royal Navy nuclear submarine may have been heading for Florida.

The £17million unarmed weapon could have been on course for the Sunshine State, MPs were told.

And it is “reasonably certain” there was “some sort of failure” in its guidance system, a military expert admitted.

Armed, the Trident II missile is capable of wiping out millions of people.

(Image: Google) (Image: Google)

Even without its deadly warheads, the 44ft 6in weapon can inflict widespread damage and kill civilians in a populated area.

The Royal United Services Institute think tank’s former director, Professor Michael Clarke, said weekend reports revealing the missile flop did not tell the full story.

The US-built missile was fired from HMS Vigilant, a Vanguard-class submarine carrying out a routine test off the US coast.

The 18,030mph Trident II D5 weapon was meant to hit a practice target in the Atlantic off West Africa.

But a “fairly major” failure of telemetry may have sent if off course, the Commons Defence Committee heard.

No 10 covered-up the missile flop, which came a month before a crunch Commons vote on renewing the deterrent.