By Westminster multimedia producer Elisa Menendez
People queuing to see the Queen Lying in State could be waiting for more than 30 hours, and stewards could close the line early if it gets too long, ITV News understands.
Officials are expecting hundreds of thousands of mourners to line up to pay their respects to Her Majesty in Westminster Hall before her funeral on Monday, 19 September.
Some suggest the number of those allowed into the historic room could be capped at around 350,000, over the period from Wednesday evening to Monday morning.
A government spokesperson for the department handling the logistics told ITV News that stewards may have to turn people away if they are unlikely to get into the Hall before the Queen's coffin is moved for her funeral.
"Common sense" will have to be used at some point if too many are joining the back of the queue to avoid them waiting for hours in vain, said the spokesperson for the Department for Digital, Culture, Media and Sport (DCMS).
However, live updates will be provided regularly online showing wait times and detailing how how far back the queue is stretching.
The management of the queues outside is codenamed Operation Feather.
"Cultural organisations" running along the route are planning to open their facilities to people in the queue, with cafes and kiosks inside selling refreshments throughout the nights.
Private cafes and restaurants are also expected to stay open, with suggestions some may choose to keep doors open for 24 hours as people queue overnight for the once-in-a-lifetime occasion.
Around 200,000 mourners queued for miles along London Bridge and right down the Southbank to visit Queen Elizabeth, the Queen Mother, when she Lay in State in 2002.
Mourners have been told to prepare appropriately for the lengthy queues, while officials in London have issued guidance and rules on how to pay tribute to the monarch respectfully.
Visitors have been urged to plan ahead for "unprecedented travel demand" around the capital with delays to public transport, closed Tube stations and road closures expected.
Those wishing to pay their respects will be able to file past Her Majesty's coffin 24 hours a day from 5pm on Wednesday, September 14 until 6.30am on the day of her funeral – Monday, September 19.
The closed coffin will be draped in the Royal Standard with the Orb and Sceptre placed on top, as it rests on a raised platform, called a catafalque.
Earlier on Tuesday, the Scottish Government announced the queue to see the Queen Lying at Rest had closed an hour and a half before visits to Edinburgh's St Giles' Cathedral were due to end at 3pm.
Around 26,000 have paid their respects to the late Queen since the cathedral opened to the public at 6pm on Monday.
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Her Majesty's coffin will shortly be moved to Edinburgh Airport and flown to London, where it will be taken to Buckingham Palace.
Its arrival will be witnessed by King Charles before resting in the Bow Room. A rehearsal for the procession of the coffin from Buckingham Palace to the Palace of Westminster will also take place.
The Queen's Lying in State will begin in Westminster Hall on Wednesday morning following a ceremonial procession through London, with the public allowed inside from 5pm.