Britain's longest serving monarch, the 96-year-old queen, has been unwell for over a year now. “Operation London Bridge" is Britain’s not-so-secret plan for how to proceed in the event Queen Elizabeth II dies. When the unfortunate event happens, the prime minister, Liz Truss, will be informed in coded terms by civil servants who will say “London Bridge is down" on secure lines.

From there the news will be sent from Foreign Office’s Global Response Centre in London to 15 governments outside the UK where the Queen is also the head of state, and the 36 other nations of the Commonwealth for whom she has served as a symbolic figurehead.

As in Buckingham Palace, a footman in mourning clothes will emerge from a door, cross the dull pink gravel and pin a black-edged notice to the gates. While he does this, the palace website will be transformed into a sombre, single page, showing the same text on a dark background.

There will be tweets, reels, posts, stories, breaking news.

If and when the Queen dies, both houses of parliament will be recalled, people will go home from work early, and airplane pilots will announce the news to their passengers.

In the nine days that follow (in London Bridge planning documents, these are known as “D-day", “D+1" and so on) there will be ritual proclamations, a four-nation tour by the new king, redacted television programming, and a diplomatic assembling in London.

After the announcement all government business will come to a halt, with the U.K. parliament, as well as its devolved legislatures in Northern Ireland, Scotland, and Wales, being adjourned. In Whitehall flags will be flown at half mast and the prime minister will make a statement and a national minute of silence will be announced. A service of remembrance is expected to take place at St. Paul’s Cathedral in London, with the prime minister and other senior ministers in attendance.

On the same day, the plan for the ascension of Prince Charles to the throne—dubbed Operation Spring Tide—will be underway. The Prime Minister will hold an audience with Prince Charles, who will then deliver his first broadcast to the nation.

The funeral is expected to take place 10 days after the Queen’s death. Should that happen at her Balmoral estate in Scotland, where the Queen is currently, her coffin will be transported down to London by royal train (dubbed Operation UNICORN) or by plane (Operation OVERSTUDY), where it will be received by the prime minister and other senior government figures.

“The Queen is Britain’s last living link with our former greatness – the nation’s id, its problematic self-regard – which is still defined by the victory in the second world war,"

The monarch's death will release its own destabilising forces. These include:

-The accession of Queen Camilla

-The optics of a new king, Prince Charles, who is already an old man

- Future of the Commonwealth

London Bridge is Queen Elizabeth II’s exit plan. It will be 10 days of sorrow and spectacle.

The most elaborate plans are for what happens if the monarch passes away at Balmoral. Queen Elizabeth II spends three months of the year at the summer home in the Scottish highlands.

This will trigger an initial wave of Scottish ritual. First, the Queen’s body will lie at rest in her smallest palace, at Holyroodhouse, in Edinburgh, where she is traditionally guarded by the Royal Company of Archers, who wear eagle feathers in their bonnets. Then the coffin will be carried up the Royal Mile to St Giles’s cathedral, for a service of reception, before being put on board the Royal Train at Waverley station for a sad progress down the east coast mainline.

Crowds are expected at level crossings and on station platforms the length of the country – from Musselburgh and Thirsk in the north, to Peterborough and Hatfield in the south – to throw flowers on the passing train. (Another locomotive will follow behind, to clear debris from the tracks.)