A police officer on duty when the Queen died has told of their emotion as the dramatic message came over the radio.
On Twitter, @AnotherMetPC said: "Tonight as I was on duty, I heard the following circulation over the radio, ''All units standby, London Bridge has fallen down, London Bridge has fallen down.
"'Standby for further direction, Met control out'."
The codename 'London Bridge' has been used to denote the Queen and is one of many used for Royals and high-ranking politicians.
The officer, who does not use their real name but reveals they are a police constable aged 25, added that they were "a bit emotional" after hearing the message while on duty.
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The tweet has quickly gone viral, with another Twitter user replying: "Of all the tweets I am reading this has made me cry the most. A message no one ever wanted to hear."
Operation London Bridge is the official plan for the days following the Monarch's death.
Last year full details of Operation London Bridge were leaked, giving a full day-to-day run down of what will happen when the Queen passes away.
Today is known as D Day, with the days following called D Day plus the number of days that have passed.
For example the second day will be D Day Plus One.
The official news of the Queen's death came from Buckingham Palace yesterday afternoon.
It read: "The Queen died peacefully at Balmoral this afternoon.
"The King and The Queen Consort will remain at Balmoral this evening and will return to London tomorrow."
King Charles III has announced 17 days of official mourning for his beloved mother and the Queen's funeral is expected to take place on Monday, September 19.
Government departments were instructed to have flags at half mast within 10 minutes of the announcement, with Cabinet members set to be called with the sad news and urged to show "discretion".
You can leave your tributes to Queen Elizabeth II here
