Mourners could have to queue for 30 hours to see the Queen lying in state in Westminster, Tory ministers believe.
Culture Secretary Michelle Donelan today warned Tory MPs of the huge number of mourners expected to descend on London for the four-and-a-half day vigil.
She told them in a WhatsApp group: “Queues could be up to 30 hours as we are obviously expecting and planning for unprecedented demand."
An exact "opening time" for the queue has not been released and it was still being set up on Monday morning, with officials preparing to lay out portable toilets and refreshment stands along the route.
Sources told The Times they are expecting 750,000 mourners, outstripping the 200,000 who saw the Queen Mother in 2002, in a five-mile queue.
But it could be more and “there’s just no way of knowing”, one source told the newspaper.
The government is said to be braced for London to become “full” for the first time, with 1,500 soldiers to help stewards man the queue, and 10,000 police officers.
The start of the queue line had already been set up by 7am today.
Private security guards in fluorescent vests were stationed at regular points along the empty line, which had yellow arches and crowd control barriers.
It stretched from Parliament’s tourist entrance Cromwell Green down Millbank, past the Lords and Victoria Tower Gardens, across Lambeth Bridge and looped back north on the other side of the river, towards St Thomas' Hospital.
No one was yet in the queue at 7am, and government officials are asking people not to join the queue for the lying-in-state yet.
Instead, they should only do so after full details of the route have been released at 10pm on Tuesday.
The queue will be open 24 hours a day from 5pm Wednesday to 6.30am Monday.
The Sun reported it will stretch for three miles along the Thames' South Bank to a point near Tower Bridge, with waits potentially reaching 12 hours.
But the Times reported it could stretch even further to Southwark Park, with fears the peak wait time will be 20 hours.
Ms Donelan then warned of a 30-hour wait in a message to Tory MPs.
One source admitted “diehard royalists” might join earlier than Wednesday, but urged them not to do so as the facilities for them are not yet in place.
People are urged to bring weatherproof clothing and a portable phone charger when the queue does open.
Mourners are warned they will not be able to take large bags, food and drink or tributes into the Palace, and the queue is "expected to be very long.
“You will need to stand for many hours, possibly overnight, with very little opportunity to sit down as the queue will be continuously moving."
At Cromwell Green entrance a large marquee had been erected over a car park this morning to help process the vast numbers of people expected to arrive.
Parliament’s Carriage Gates and other entrances were already shut to passholders on Monday morning as security ramped up across the estate.
Portcullis House - normally silent and empty at 7am - was bustling with police, security, and a small number of MPs in mourning clothes.
The coffin will rest on a raised platform - known as a catafalque - under the hammer beam roof of Westminster Hall, the oldest part of the Palace of Westminster.
The late monarch's casket will be draped by the Royal Standard with the Orb and Sceptre placed on top.
Each corner of the platform will be guarded around the clock as units from the Sovereign's Bodyguard, the Household Division, or Yeoman Warders of the Tower of London stand in solemn vigil.
Visitors will have to go through airport security-style checks to enter the grand hall, which is one of the only remaining parts of the ancient Palace of Westminster.
Only small bags (40cm x 30cm x 20cm) are allowed and mourners are advised to bring food and drink, portable mobile chargers and umbrellas for the queue.
Flowers and other tributes are banned, as are banners and flags.
People are asked to dress appropriately and not to wear clothes with offensive or political slogans.
Taking photos and videos is not allowed within the Palace of Westminster.
The Queen began her final journey from Balmoral in the Scottish Highlands yesterday, with crowds of people lining the roads as her coffin was borne on the 180-mile trip south to Edinburgh.
Her coffin was placed to rest in the Palace of Holyroodhouse overnight. Today the Queen's coffin will travel in a procession to St Giles’ Cathedral along the Royal Mile with King Charles and the late Queen’s other children Anne, Andrew and Edward following behind on foot.
The late monarch will lie in state for 24 hours in Edinburgh from 5pm today. Her coffin will then be taken to London on Tuesday on an RAF plane.
The solemn procession will then travel to Buckingham Palace by road, where she will rest in the Bow Room.
On Wednesday afternoon, her coffin will be taken on a gun carriage of The King’s Troop Royal Horse Artillery from Buckingham Palace to the Palace of Westminster.
