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After the bewildering experience of being anointed by the Queen at Buckingham Palace and stepping over the threshold of 10 Downing Street, Mrs May had the sobering task of preparing for nuclear war – if the unthinkable ever happened.

For Britain"s new Prime Minister was tasked with writing what is known as "a letter of last resort".

This doomsday note is to be read by captains of the UK's Trident submarine fleet and gives the order to strike back if Britain has been wiped out.

The Royal Navy has four submarines which are always armed and one or more are always under the sea in case their base in Faslane on the Clyde has been nuked.

Each sub carries 16 Trident II ballistic missiles with a range of 7,000 miles and 12 independently-controlled warheads.

They are capable of destroying a large city in an instant.

Mrs May has to write the letter just in case she is killed in a nuclear blast or cannot give orders to sub commanders.

Chief of defence staff Sir Nicholas Houghton will have met her and both will have sat down to do the most important job she will ever do as Prime Minister.

(Image: PA)

The options are said to include:

Copies of Ms May's letters are then put in a secure box and transported under guard to Trident submarines – where they will be placed into the boat"s safe.

Only the senior commander will be able to open the letter, which are essentially orders to nuclear bomb enemy targets.

David Cameron's letter of last resort will already have been destroyed by the time May writes hers.

Witnesses describe Tony Blair going very quiet and pale when writing his.