Who, What, Why: Is there such a thing as a nuclear button?

Labour leader Jeremy Corbyn has said he wouldn't press a nuclear button. But does one actually exist, asks Justin Parkinson.

It's an evocative image. Obama, Putin, Hollande, Cameron and others sitting in front of a single, usually red, button, with which they can unleash their nuclear arsenals.

When asked on BBC Radio 4's Today programme whether he would ever push such a contraption if he became prime minister, Labour leader Jeremy Corbyn replied: "No." But he needn't worry about the button, at least. There's no such thing.

While the ultimate decision on whether to launch is the prime minister's (or that of a nominated deputy while he's away from the UK), the order has to go through several protocols, says defence expert Paul Beaver. A sequencing system ensures a printed code, stored in a secret Ministry of Defence location, has to tally with that kept in a safe on board the nuclear submarine.

Two officers, extensively tested for their mental stability, sit in separate parts of the submarine and enter the code simultaneously into a computer, allowing the launch.

The PM makes the decision, but several other people probably get involved at various points during the process, including the attorney general and the chairman of the Joint Intelligence Committee, says Beaver.

Within a few days of taking office, the PM writes a letter, external, also to be stored in the submarine's safe. It states whether, in the event of his or her death (and that of their nominated deputy) in a nuclear attack, there should be a retaliation. It's the submarine commander who must carry out the order.

Nine countries have nuclear warheads, totalling 16,300 altogether, according to the Stockholm International Peace Research Institute, external. The processes in place for using them, for security reasons, remain secretive.

"I wouldn't be at all surprised if there's an agreement between nations on the protocol," Beaver says. "There certainly will be between the UK, US and France."

The single, all-destructive button doesn't exist but it has a cultural resonance, as a metaphor in the machine age for getting things done instantly. As early as the 1890s a Parisian newspaper published a jokey story about the inventor Thomas Edison destroying England by connecting some wires and pressing a button, according to writer Spencer Weart.

And, a skit on the 1980s ITV show Spitting Image jokes about US President Ronald Reagan having two buttons by his bedside. He tries for the one labelled "nurse", but presses another labelled "nukes". That's unlikely to happen, anywhere.

Subscribe to the BBC News Magazine's email newsletter, external to get articles sent to your inbox.