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Foreign leaders and their spouses attending state have been told to fly commercial into and take laid-on buses to reach the service, reports have said.

Around 500 foreign dignitaries are expected to attend Britain's first state funeral in six decades as officials mount "a huge-scale operation" for the event on 19 September, a source at the Foreign Commonwealth and Development Office told AFP.

"The logistics have been meticulously planned to try to make it as smooth as possible for those attending," the source added.

News website Politico reported that attendees have been told not to use their own vehicles to reach the service at Westminster Abbey or travel across London by helicopter.

Instead, they will be transported by escorted private buses to the abbey from a site in west London where they will have parked, it reported, citing an official protocol message sent to overseas embassies.

"Tight security and road restrictions" were cited by Politico as the reasons, but 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 –Ìýtypically one of two customised Boeing 747 planes –Ìýand then use their Marine One helicopter and an armoured limousine dubbed "The Beast" to get around.

The US embassy in London did not immediately respond to a request 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 Britain has ever staged.

Thousands of people are expected to line up for the chance to see Queen Elizabeth II's flag-draped casket at St Giles' Cathedral in the Scottish capital

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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 can attend the lying-in-state and sign a condolence book at Lancaster House immediately afterwards, according to the leaked document quoted by Politico.

They will also be able to deliver a tribute to the late queen of up to three minutes, which will be recorded for the media, it said.

will reportedly host a reception at Buckingham Palace for overseas leaders on the eve of the funeral.