THE captains of Britain’s nuclear submarines had a bit of a wake up call today - when the BBC mysteriously went off air for 15 minutes.

The Today programme, which is popular with government ministers, went silent just before the 8 o’clock news because of a fire alarm at BBC HQ.

Culture Minister Tessa Jowell was speaking when the programme was cut off and the main news was replaced by classical music.

The unusual event was a crucial test for Britain’s four Trident nuclear submarines, patrolling in secret locations around the world and cut off from base.

On board are handwritten letters from Prime Minister Tony Blair, bearing his instructions for nuclear retaliation. They are locked in the submarines' safes.

Secret orders to the captains say that these deadly instructions are to be opened and acted upon only if the submarine cannot tune in to Radio 4’s Today programme for a given number of consecutive days. That is a reliable sign that Britain has been hit by a nuclear attack.

Smoke

Reports suggest that the Prime Minister’s letter, written just days after he won the 1997 election, offers the submarines four options: to put the vessels under US command, to make their way to Australia, to launch a nuclear strike against the enemy or to use their own judgement.

There are reports that Tony Blair "went white" when told he must write such letters after his election victory.

The submarine captains will have been alerted when the Today programme mysteriously went off air, even for just 15 minutes. The Ministry of Defence today refused to discuss details of instructions given to the nuclear submarines for security reasons.

After smoke was spotted at BBC TV Centre the system was shut down and Today staff rushed to another part of the building to continue the programme.

Culture Secretary Tessa Jowell who was cut off mid-sentence when the problems struck the flagship news show joked that it was the first time she was allowed to answer questions without being interrupted.

She was followed by Foreign Secretary Jack Straw who was interviewed when the show was moved to another studio. He joked: "Perhaps it’s a not so subtle bid by the BBC to increase the licence fee to pay the electricity bill."

The suspected powercut hit both Radio 4 and Radio 5 and TV’s BBC News 24 was also cut off for a short period of time.