World leaders due to attend the Queen's state funeral have reportedly been asked to fly to the UK on commercial flights and travel to the service by coaches.
Foreign Office documents obtained by Politico, which Sky News understand to be correct, also outline how heads of state have been told they cannot travel between airports and venues by helicopter "due to the number of flights operating at this time".
World leaders, including US President Joe Biden, are preparing to travel to the UK for the historic funeral at Westminster Abbey on Monday 19 September.
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The documents also outline how heads of state and their partners will arrive at Westminster Abbey in escorted coaches from a location in west London on the day of the funeral.
They will be required to leave their own cars at the site "because of tight security and road restrictions".
The Foreign Office has warned "where possible" leaders should arrive in the country on commercial flights.
The department also warned London Heathrow airport will not be available for private flight arrangements or aircraft parking.
Those traveling by private jet should head for "less busy airports" around London, the document adds.
Meanwhile, passengers using UK airports have been warned "unforeseen events may require commercial and private flights to divert from the intended airport of arrival".
Queen Elizabeth's funeral will be one of the most significant international events hosted by the UK in recent times.
Westminster Abbey will be so packed for the event it will only be possible for a single, senior representative from each country and their partner to attend, according to the documents.
World leaders who are unable to attend may choose someone else as their official representative, such as a head of government or senior minister.
King Charles III will host a reception for all the overseas leaders at Buckingham Palace on the evening before the funeral service.
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Visiting heads of state will be able to attend the lying in state of the Queen's coffin before signing the condolence book at Lancaster House immediately afterwards.
Foreign leaders will also be able to deliver a tribute to the Queen lasting up to three minutes, which will be recorded for the media.
Following the service, they will be escorted to Dean's Yard, still within the grounds of Westminster Abbey, to attend a reception hosted by Foreign Secretary James Cleverly.
They will then return to west London by coach to collect their cars.
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The White House has confirmed Mr Biden will be attending the funeral with his wife Jill.
European Commission President Ursula von der Leyen and New Zealand's Prime Minister Jacinda Ardern are also due to attend.
Japan's Emperor Naruhito, Turkey's President Recep Tayyip Erdogan, and French President Emmanuel Macron are expected to be at the service, while Spain is likely to be represented by King Felipe VI.