The security operation in the wake of the Queen's death and surrounding her funeral is likely to be the biggest the UK has ever seen, according to a former counter-terror chief.

Nick Aldworth said police and security services will anticipate huge crowds of people flocking to mourn the loss of the monarch, with plans on an "enormous" scale set in motion.

The former counter-terrorism national co-ordinator, who used to have responsibility for such events and oversaw the armed police operation in central London during the 2012 Olympics, said the plan on how to keep the public and dignitaries safe has been years in the making and is regularly revised.

The UK's terrorism threat level is currently "substantial", meaning an attack is "likely".

Mr Aldworth told the PA news agency: "It's probably the biggest operation that we're likely to mount in the UK."

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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 on police and the security operation".

While the Queen lies in state the "planning assumption" is that there will be "enormous queues of several hours long of people wanting to pay their respects".

Laying down road barriers will be one of the first tasks in anticipation of crowds lining the streets to see the 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.

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Another "major consideration" that "really stretches policing" is organising the armed protection for visiting foreign dignitaries.

"There are only so many armed protection officers in the UK," he said, warning that there are "certainly not enough in London" to manage the potential more than one hundred heads of nation states which could come to the UK.

"The whole operation will draw massively on resources from across the country," he added.

Police and security services will oversee the plans from a central operations room in Lambeth in London, with the Metropolitan Police assuming overall responsibility and other forces taking charge on activity in their areas, Mr Aldworth said.

A senior police officer, known as a gold commander, will head up the operation.

An intelligence cell would be set up, with the Met speaking to police intelligence operations and the security services.

The central operations room will be divided into sections looking at things such as the armed response, protection teams, intelligence, roads, queues, and landmarks such as Parliament and Buckingham Palace, he said.

The Duke of Edinburgh's funeral last year "took a very different profile" because of the constraints in the country at the time dictated by restrictions during the coronavirus pandemic.

"One can't rule out in the current world that the same wouldn't happen with Her Majesty's funeral," Mr Aldworth added.