Hundreds of thousands of mourners are expected to file past the Queen's coffin to pay their respects to Britain's longest reigning monarch.
The Government is bracing for unprecedented numbers of people to visit Parliament where the late Queen will lie in state for four days before the state funeral on Monday.
Culture Secretary Michelle Donelan told Tory MPs via WhatsApp that visitors could be f orced to queue for up to 30 hours to enter Westminster Hall to view the Queen's coffin.
The army is on standby to assist with policing the lines, with 1,500 soldiers ready to assist and around 10,000 police officers expected to be deployed.
Downing Street confirmed today that people in the queue will be able to leave to use portable toilets or buy food and come back to the same place, as they will be given wristbands to hold their place.
“There is a capability for people to go and use restrooms and return to the queue," the PM's spokesman said.
There will also be food and drink stands en-route, though food and drink will not be allowed into Westminster Hall, which will have airport-style security and only small bags allowed.
The spokesman warned people to plan ahead as it will be "extremely busy" - and numbers are expected to dwarf the 200,000 mourners who visited the Queen Mother as she lay in state in 2002.
Commuters may want to "change their working patterns accordingly" as London will be "extremely busy" for the Queen's lying in state, No 10 said.
Asked if Londoners who don't want to see the coffin should work from home, the PM's spokesman said: "London will be busy so some people may wish to change their working patterns accordingly.
"But not everyone will be able to do that.”
Asked if there will be any facilities for people who physically cannot queue for 30 hours, he said: "Obviously we want everyone to be able to attend regardless of whether they have disabilities.
"Our focus is on ensuring they have the information needed to make the decision about what's right for them.
"There will be toilet facilities, there will be first aid available, there will be the ability for people obviously to go and use toilets and return to queues and things like that."
The first person to begin queuing for the Queen's lying in state in London said she "really, really wanted to be part of it".
Vanessa Nathakumaran, 56, from Harrow, arrived at noon on Monday - more than 48 hours before lying in state begins.
She told PA she started admiring the royal family in her childhood in Sri Lanka before she moved to the UK in the 1980s.
She said: "I really really want to be part of it", adding: "She has done a good service to our country."
Crowds have been queuing to pay their respects to the monarch in Edinburgh where she will lie in state in St Giles's Cathedral for 24 hours.
The Queen's coffin will arrive in London on Tuesday night and will rest in Buckingham Palace overnight before she is be borne to Westminster Hall on Wednesday afternoon.
The public will be able to view the coffin 24 hours a day from 5pm on Wednesday until 6.30am on Monday, September 19, the day of her funeral.
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.
Mourners are warned they will need to stand for many hours, possibly overnight, with very little opportunity to sit down as the queue will be continuously moving."
Large bags, food and drink, and tributes cannot be taken inside. Photography and video is also banned.
