Foreign leaders and their spouses attending Queen Elizabeth II's state funeral have been asked to arrive in Britain on commercial flights and use coaches provided to reach the service, reports have said.
Only heads of state and their spouse, or another senior representative and their partner, have been invited to the first state funeral in the UK in nearly six decades, according to news website Politico.
Attendees have been told not to use their own vehicles to reach the service at Westminster Abbey, Politico reported on Sunday.
It cited an official protocol message sent to overseas embassies by the Foreign Commonwealth and Development Office (FCDO).
Instead, they will be transported en masse on private coaches to the central London abbey. They cannot use options such as helicopters to cross the British capital, said the official document, which was reportedly sent out on Saturday.
The FCDO "regrets that, because of limited space at the state funeral service and associated events, no other members of the principal guest's family, staff or entourage may be admitted", it added.
The purported stipulations have raised eyebrows, with one London-based foreign ambassador telling Politico: "Can you imagine Joe Biden on the bus?"
US presidents travel long-distance on Air Force One aircraft -- typically one of two customized Boeing 747 planes -- and then use their Marine One helicopter and an armoued limousine dubbed "The Beast" to get around.
The FCDO and US Embassy in London did not immediately respond to requests for comment.
The state funeral, the first since that of former prime minister Winston Churchill in 1965, will be one of the biggest security and logistical operations the UK has ever staged.
Thousands of police officers from across the country will be redeployed to London to help manage the huge crowds expected.
As well as world leaders and royals, up to a million people are expected to descend on the streets of capital during the four-day period leading up to the funeral, when the queen's coffin will lie in state at parliament.
Visiting global leaders will be able to attend the lying-in-state and sign a condolence book at Lancaster House immediately afterwards, according to the leaked Foreign Office dispatch report.
They will also be able to deliver a tribute to the late queen lasting up to three minutes, which will be recorded for the media, it said.
King Charles III will reportedly host a reception at Buckingham Palace for all the overseas leaders attending, on the evening before the funeral.
On the day itself, the visiting dignitaries will arrive at Westminster Abbey in escorted coaches from a site in west London where they will have parked.
"Tight security and road restrictions" were cited as the reasons f