The UK's nuclear arsenal is housed on four submarines, with one of those submarines on patrol at all times.

During their isolated missions, crews watch for signals that the UK still exists — and may launch a counter-attack if they believe their country has been destroyed.

One of these signs is whether BBC Radio 4 is still broadcasting the "Today" programme, Britain's flagship news and politics show.

If the submarine commander believes Britain has been destroyed, he may be under orders to launch a nuclear strike.

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Deep underwater, on submarines equipped with nuclear missiles, British crews are constantly prepared to fire their weapons, and potentially play a part in bringing about the end of the world.

Sailors on the four Vanguard-class submarines which patrol the waters and hold the UK's nuclear deterrent operate under strict protocol for working out when to act and what to do — part of which is said to include listening to BBC radio.

According to a prominent British historian, the broadcast of BBC Radio 4's "Today" programme is one of the official measures the Royal Navy uses to prove that the United Kingdom still exists. "Today" has been broadcast at around breakfast time since 1958 and is the highest-profile news programme in British media.

Commander Dan Martyn stands beside HMS Vigilant, one of the UK's four submarines carrying nuclear missiles, in 2016. Jeff J Mitchell/Getty Images

Lord Peter Hennessy, a history professor who joined the UK's House of Lords in 2010, said that if it can't be heard for three days in a row, then it could signify Britain's demise, and trigger the doomsday protocol.

According to Politico, Hennessy says: "The failure to pick up the BBC Today program for a few days is regarded as the ultimate test."

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If no sign comes through, the commander and deputy will open letters that contain instructions from the prime minister and execute their final wishes.

These letters, each known as a "Letter of Last Resort' are secret instructions, written when a prime minister enters the office and sealed until an apocalypse. They tell the UK's submarine commanders what to do with the country's nuclear weapons if the entire British chain of command has been destroyed.

Writing these letters is one of the first tasks undertaken by any new prime minister. They are locked inside a safe inside another safe, and placed in the control rooms of the nation's four nuclear submarines, Politico reports. The safes will only be accessible to the sub's commander and deputy.

Matthew Seligman, Professor of Naval History at Brunel University, told BBC Newsbeat that there are "only so many options available."

"Do nothing, launch a retaliatory strike, offer yourself to an ally like the USA, or use your own judgment.

"Essentially, are you going to use the missiles or not?"

One of the UK's Vanguard-class submarines. Royal Navy

The UK has four submarines that are capable of carrying the country's Trident nuclear missiles. At least one of these has been on patrol at all times since 1969, the government says.

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There are 40 nuclear warheads and a maximum of eight missiles on each submarine. Their systems are "so sensitive they can hear vessels over 50 miles away."

Only the prime minister can authorize the launch of the country's nuclear weapons.