A former counter-terror police officer has warned that it “just takes one person to do something abhorrent” as security preparations intensify for the Queen’s lying in state.
People have already started queuing to pay their respects at Westminster Hall, although access will not start until Wednesday evening.
The government said visitors would face “airport-style” security before entering parliament and police will work to protect the queue, which is expected to be many hours and thousands of people long.
Nick Aldworth, who led the “protect and prepare” strand of national counter-terrorism policing until his retirement in May 2019, said there was a threat of lone-actor terror attacks such as those that previously hit crowded London targets using vehicles and knives.
“It just takes one car, one person to do something abhorrent and not only have you disrupted a constitutional event, people will be injured and killed,” he told The Independent.
“Operation London Bridge has its genesis in a time that is no longer relevant [to the current terror threat] and my caution to the Metropolitan Police would be to remember that.
“It’s not just terrorist risk, there are many organisations that are slightly anarchic - what happens if 40 people glue themselves to a road when a hearse is on the way?”
Scotland Yard said it had initiated “well-rehearsed policing plans”, with new commissioner Sir Mark Rowley saying he was involved in planning for the Queen’s death during his previous role as the head of national counter-terrorism policing.
“They have been working with colleagues from across government and the Royal Household and others, as you would expect,” he told Sky News.
“We will have a safe event but we will be putting thousands of officers into this because of the level of security required and the millions of people who want to pay their respects.”
Large areas of Westminster around the route the Queen’s coffin will take on Wednesday have been closed, and the queue to see her lying in state will also be protected by “hostile vehicle mitigation” measures.
“Visitors will go through airport-style security and there are tight restrictions on what you can take in,” guidance for the public states.
“The police may conduct security searches along parts of the queue.”
Measures mirroring those brought in for international flights following 9/11 and a series of terrorist plots aiming to blow up planes will see people prevented from taking liquids inside the Palace of Westminster.
Other banned items include large bags, knives and sharp items, “personal defence sprays”, flowers and tributes.
Restrictions targeting potential protesters include a prohibition on banners, flags, climbing gear, padlocks, fireworks, paint, flares and “other items that could be used to cause a disturbance or noise”.
Several protesters have been arrested at events linked to the Queen’s cortege and proclamation of the new King in recent days, sparking a backlash from human rights groups.
The Queen’s coffin will lie in state for several days ahead of her funeral on Monday, which will be attended by US president Joe Biden and dignitaries from around the world.
An intensive security operation is being mounted for the international delegations, high-profile attendees from the royal family and British public life, as well as the crowds expected to gather in London.
A significant armed police presence will be in place for the state funeral, including rooftop snipers guarding the procession and patrols on the ground.
Officers are being drawn from forces across the UK under “mutual aid” arrangements, causing the cancellation of football matches because of a lack of public order-trained officers.
A spokesperson for the National Police Chiefs’ Council said: “Police forces have business continuity plans in place to ensure they are prepared to assist with this operation, in addition to continuing their core service to the public, keeping communities safe.”
