Why Biden won't be riding the bus to Queen's funeral

US President Joe Biden will be exempt from a rule that every other world leader at the royal funeral tonight will be subject to.

As world leaders and dignitaries assemble in a "remote" London location to board coaches for Westminster and the official service to farewell Queen Elizabeth II, Biden will be able to travel in privacy.

The presidential motorcade will travel separately to the coaches, after US officials shot down reports Biden would be riding the bus.

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For security reasons, Biden will be ensconced in "The Beast" - the presidential limousine.

The car, manufactured by General Motors, is specially armoured and is also equipped to resist gas attacks.

It even carries packs of the president's blood type in case of a medical emergency.

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Despite the special treatment, Biden has been vocal in his respect for the Queen, describing her as "decent, honourable and all about service".

"We've had an opportunity to meet with an awful lot of consequential people. But I can say that the ones who stand out in your mind are those whose relationship and interaction with you are consistent with their reputation," Biden said after signing a condolence book overnight.

Biden and the Queen first met in 1982, when as a young senator, Biden's own Irish-American mother instructed him: "Don't you bow down to her".

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He didn't bow down then, or when he met the Queen as president last year while attending a Group of 7 summit in England.

But he made plain his regard for her as a leader.

"She was a great lady. We're so delighted we got to meet her," Biden said on the day that she died.