'Operation Unicorn' plans were drawn up years ago outlining the procedures for if the Queen were to die in Scotland.
The Queen is currently under "medical supervision" following concerns for her health as members of the Royal Family have travelled up to Balmoral to be with her.
Harry was expected to make a speech at the ceremony this evening, but now he and Meghan Markle are making their way to Scotland.
A Palace spokesperson said: "Following further evaluation this morning, the Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision. The Queen remains comfortable and at Balmoral."
Plans have been ready for some time in the event of the death of the monarch who has been on the throne for 70 years and they change depending on where the Queen passes away.
Operation Unicorn is ready if the country's longest-reigning monarch dies in her favourite residence, which will likely see the neighbouring palace of Holyroodhouse and St Giles' Cathedral as the main places for the public to pay their respects.
According to the plans, during a period of mourning, her body will be moved from Balmoral to Holyroodhouse in Edinburgh before travelling to London via train.
If she travels by air, the operation will be known as "Operation Overstudy", according to plans leaked to Politico in 2021.
It is understood that her body will be placed on the Royal Train at Waverley Station and will travel on the east coast mainline before finishing in London.
A 10-day mourning period will be in place up to and including the Queen's funeral.
More famous is 'Operation London Bridge', which is applied if the Queen dies in London.
Concerns among the public and MPs have been visible since news broke of doctor's concerns around midday today.
Prime Minister Liz Truss said: "The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime. My thoughts - and the thoughts of people across our United Kingdom - are with Her Majesty The Queen and her family at this time."
Leader of the opposition Sir Keir Starmer added: "Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon.
"My thoughts are with Her Majesty The Queen and her family at this time, and I join everyone across the United Kingdom in hoping for her recovery."
