Former Metropolitan Police commander Bob Broadhurst, who led the policing operation for the London 2012 Olympics, said all leave and training was likely to be cancelled and extra help called in from surrounding forces in the Home Counties.

The Met will also need extra specialist officers from outside forces across the UK, including close protection and firearms, to take part in the meticulously planned and complex security operation.

During the Olympics and Paralympics in 2012, around 10,000 officers were on duty in London per day, and Mr Broadhurst, who was with the Met for 36 years, expects similar numbers in the lead up to the Queen’s funeral.

Police officers wait outside Buckingham Palace in London. Picture: AP Photo/Christophe Ena

The largest annual policing event for the Met is usually the Notting Hill Carnival, during which around 6,000 officers are on duty per day.

Mr Broadhurst said demand would build up as the days go on, with larger crowds gathering in London.

He said: “Clearly you need to manage crowds. Unfortunately large numbers of people also bring in criminal opportunities, so you have to manage crime.

“You have to manage the basics like lost children, all the other stuff that goes with that.

“Clearly terrorism will be a factor, security is a big issue right from the outset.

“Not necessarily the more organised terrorist groups, but lone individuals, that has to be factored in as well.”

Mourners are expected to queue for several hours, including overnight, to pay their respects to the Queen as she lies in state, expected to be from Wednesday.

Members of the public waited up to ten hours to see Diana, Princess of Wales and the Queen Mother lie in state following their deaths.

The Met will also be responsible for the protection of public figures and foreign dignitaries who attend the funeral, and dealing with any protests that their visits may trigger.

Mr Broadhurst said: “By the end of the week you start to click in to what will be, I would say almost certainly, the biggest security operation the country has ever seen.

“Practically every nation on earth is going to want to send their king, queen, prime minister or president for the funeral.”

Former counter-terrorism national co-ordinator Nick Aldworth said tens of thousands of people arriving around the clock over several days to line routes ahead of any ceremonial event would create the most demand in policing and security terms.

Installing road barriers will be one of the first tasks in anticipation of crowds lining the streets to see the Queen’s funeral cortege pass by.

There will be a “significant armed operation”, with officers on patrol and queues formed in a way so people are not crushed and lorries and cars cannot be driven into them.

Rooftop snipers will be in place while the cortege is moving, receiving a helicopter escort anywhere outside of London.

Police and security services will be alert to the prospect of knife attacks, bombs being detonated, and all other possible terror threats or incidents.

Mr Aldworth said: “Clearly thousands of people gathered creates an attractive opportunity for any of those methodologies,” but some threats are “easier to manage than others”.

Security screening such as bag checks to look for knives and other weapons will be carried out in some areas.

“We always think about vehicles and blunt or bladed weapons as being the most likely because they’re easy to get hold of and don’t require any skill or planning to use,” Mr Aldworth said, adding that bombs “are less likely” as they are made to order.

Preparations will also involve City of London and British Transport Police, as well as staff from the Royal Parks, Transport for London and Westminster Council.

The Metropolitan Police said “well-rehearsed” policing plans were under way.

There will be extra police officers on patrol at key locations, including transport hubs and the royal parks and residences in London.

Road closures will be put in place in parts of Westminster in advance of the ceremonial events planned around the Queen’s funeral.

Deputy Assistant Commissioner Stuart Cundy said: “I wish to pass my deepest sympathies to the entire royal family at this extremely sad time.

“This will be an extremely poignant moment for the nation, and I know people will want to pay respects and celebrate the Queen’s dedication to public service.

“Working closely with the City of London Police and British Transport Police, the Met will now co-ordinate and deploy a comprehensive policing plan in London.

“This operation will be highly visible, particularly in Westminster and areas around Buckingham Palace and St James’s Park.

“A great number of police officers will be on duty during this period, to ensure the safety of those visiting London and to deter any potential criminality.

“Officers will also be offering advice to visitors, particularly those who may be unfamiliar with the city.”

Queen's death: Around 10,000 police officers on duty per day in UK’s biggest security operation