The source said: "Internally, there is an investigation into which version has been leaked and how sensitive it is.
"If it turns out to be an old version that was widely circulated and does not include the most sensitive material, it might go no further than that – but if it's a fuller version that is only circulated to, say, 10 people, then the Cabinet Office will launch a formal inquiry."
The source said the leak would "cause concern" over security which would have to be factored into future versions of the plan, adding: "It's insensitive because it isn't long since the Queen lost her husband – it's rude and it is entirely unnecessary."
The document leaked to Politico contains some details that are not in the most up-to-date version, and is understood to be at least a few months old.
More than 40 organisations – including military units, councils, charities and broadcasters – receive copies of parts of the plan pertaining to their involvement, but only a small number of people are trusted with the whole document.
The plans drawn up for the period immediately following the monarch's death have been updated within the last 18 months to include references to the challenges that would be faced during the Covid pandemic.
Government departments have also raised concerns about the vast security operation that will be set in motion amid fears of an unprecedented influx of tourists. Concerns include the management of huge crowds in a bid to avoid travel chaos and that London could become "full".
The plan covers every eventuality, from how the Queen's coffin will be transported to London if she dies outside the capital to the specific wording of notifications sent by the departmental permanent secretaries tasked with breaking the news.
As has previously been reported, the Prime Minister and senior governors general and ambassadors will be among the first to learn of the Queen's death, known as "D-Day". Thereafter, a 10-day plan leading up to her funeral refers to each day as "D-1", "D-2" and so on.
Upon receipt of an email from the Cabinet Secretary to ministers and senior civil servants, flags across Whitehall will be lowered to half mast, with the aim for this to be done within 10 minutes. But in one document, Downing Street raised concerns that it would face public anger if it was unable to lower its flags within 10 minutes and may have to rely on an external contractor.
After the Prime Minister has made a statement, there will be gun salutes and a national minute's silence will be announced. The Prime Minister will then hold an audience with the new King and, at 6pm, King Charles will broadcast to the nation.
At the same time, there will be a service of remembrance at St Paul's Cathedral in London, attended by the Prime Minister and senior ministers. The service is planned to appear "spontaneous", it is claimed.
King Charles will then embark on a tour of the UK in the days before the funeral in line with Operation Spring Tide, the plan for his accession to the throne. The tour will start in Edinburgh, with a visit to the Scottish parliament followed by visits to Northern Ireland and Wales.
In one memo, a worst-case scenario outlines how London could become "full" as hundreds of thousands of people travel to the capital, with accommodation, roads, public transport, food supply, the police and the NHS stretched to breaking point. Concerns have also been raised about a shortage of stewards for crowd control.
The state funeral will be held at Westminster Abbey, with a two-minute silence across the nation at midday. There will also be a service at St George's Chapel at Windsor Castle, and the Queen will be buried in the castle's King George VI Memorial Chapel.
While the monarch – who marked her 95th birthday in April just weeks after the death of her husband Prince Philip at age 99 – remains in good health, the government has long planned for her death.
But according to Politico, the plans have been updated by the Cabinet Office recently and now contain specific references to the Covid pandemic.
The meticulous guidelines cover everything from managing unprecedented crowds and traffic congestion in London to the Prime Minister and Cabinet officials meeting the Queen's coffin at St Pancras Station.
Politico also revealed Prince Charles – who will become king after his mother's death – will travel across the UK in the days leading up to the funeral.
The documents cover the schedule for the 10 days after the Queen's death in painstaking detail.
Immediately after
The day of Her Majesty's death will be known as "D-Day" by officials, and according to Politico, a so-called "call cascade" will kick in soon after the Queen dies, whereby the Prime Minister will be informed followed by other senior government representatives.
An official notification will also break the news to the masses.
The flags at 10 Downing St will be lowered to half-mast within 10 minutes of the news breaking, and the royal family's official website will go black in a sign of respect, with a short message relaying the news.
The official UK government website will also display a black banner of mourning, and there will be a pause on non-essential government social media content.
The royals will soon release details of the funeral and the Prime Minister will issue an official statement, with gun salutes to be organised by the Ministry of Defence.
At 6pm on the night of his mother's death, the new King Charles will address the public, while a small service is to be held at St Paul's Cathedral for senior government officials only.
Day 1
The following day, the Accession Council will proclaim Charles the new King.
Parliament will meet to draft a condolence message, but apart from that, all other parliamentary work will halt for 10 days.
That afternoon, the PM and senior ministers will meet King Charles in person.
Day 2
The Queen's coffin will return to Buckingham Palace, with a range of specific plans in place depending on where the Queen happens to be at the time of her death.
Days 3-5
King Charles will set off on his tour of the UK.
Official tributes and condolences will continue to pour in.
Days 6-9
The Queen will lie in state at the Palace of Westminster for the three days leading up to her funeral.
Her coffin will be accessible to the public for 23 hours a day.
Day 10
The state funeral will be held at Westminster Abbey, with two minutes' silence observed across the nation at noon.
Concerns raised
While the plans contain stunning levels of detail, officials have raised a number of concerns, including the pressures which will be placed on the public transportation system and the need for crowd control, as record levels of people are expected to flock to the capital to pay their respects.
Those incredible crowds could bring with them added threats posed by the ongoing Covid pandemic, if the Queen happens to die while the virus is still circulating in the community, and authorities will also be on the lookout for any increased terror threat.
