People wanting to see The Queen lying in state may have to queue for 12 hours
Whitehall chiefs in charge of logistics for the historic five-night vigil have estimated mourner numbers could be close to that which turned up to view Pope John Paul II, according to The Sun.
A million mourners filed past the late pope when he lay in state in Rome in 2005.
While Westminster Hall will be open 24 hours a day for four days for crowds to view the Queen's coffin, "insiders" reportedly told the paper people may have to wait up to 12 hours in a three-mile-long queue.
The Rail Delivery Group has meanwhile warned those wanting to travel to Westminster Hall to expect services to London and all of its stations to be "extremely busy".
The rail operator's customer information director told the BBC people needed to plan carefully and allow plenty of time if planning to travel during the official mourning period.
Transport for London also warned that roads and public transport in central London will be very busy and has advised travellers to allow "plenty of extra time" for their journeys and to avoid driving where possible.
Andy Byford, London's Transport Commissioner, told the national broadcaster: "We are working with our partners to keep our city moving smoothly and to ensure that everyone who is planning to attend the memorial events can do so safely."
The Queen’s body is currently lying in state at St Giles’ Cathedral in Edinburgh.