'London bridge is down': Code word for Queen Elizabeth's death

The 96-year-old monarch had been down with health problems since October last year that left her with difficulties in walking and standing.

Earlier this month, leaked documents in the media space revealed the extent of a massive operation to be made active just after Queen Elizabeth II dies.

Details of "Operation London Bridge" were leaked to Politico, which said officials would refer to the day the Queen dies as "D Day".

The code for conveying the message that the Queen has died would be - "London bridge is down," said reports.

With the Queen's death, her eldest son and Prince of Wales-Charles became the new monarch.

An elaborate security operation has been already planned to manage the sea of crowds which would be overpouring as people will gather to pay their last respects to the Queen.

There would be a supposedly "spontaneous" service at St Paul's Cathedral, according to Politico, and the new King Charles would tour the four nations of the United Kingdom in the days after her death.

Buckingham Palace officials had declined to comment, either on the leak or the plans.

The Queen reigned for 70 years.

The royal family including the Queen's son and heir Prince Charles, grandsons William and Harry and their families have gathered at her Balmoral retreat in the Scottish highlands, where she spent her last days.

The UK celebrated the Queen's Platinum Jubilee to mark 70 years of service to the country with gala events in June this year.

In 2015, Queen Elizabeth became the longest-serving British monarch, surpassing her great-great-grandmother Queen Victoria. This year, she became the world's second-longest-reigning monarch.