Queen Elizabeth II has jokingly compared a noisy helicopter to those used by US presidents Donald Trump and Barack Obama during an interview with filmmaker Sir David Attenborough.

A documentary to be aired in the UK shows the Queen strolling through the garden at Buckingham Palace with Attenborough, making quips and even mentioning the prospect of her own demise in a light-hearted way.

The scene, filmed last summer for a program to be broadcast next week, offers an unusually informal glimpse of the 91-year-old monarch, who has given away few personal details during her 66-year reign.

She is seen walking and chatting with Attenborough, also 91, known for lavish series like Planet Earth and Blue Planet.

During one light-hearted scene, the pair are interrupted by the noise of a helicopter as they walk through the gardens of Buckingham Palace.

The Queen is heard asking Attenborough why helicopters tend to appear during conversations.

“Why do they always go round and round when you want to talk?” she asks.

The Queen goes on to say the noise reminded her of the aircraft used by Mr Trump, and his predecessor, Mr Obama.

"Sounds like President Trump or President Obama."

UK's Queen Elizabeth jokes about loud helicopters as she talks with wildlife documentary maker Sir David Attenborough. pic.twitter.com/My74nuA5Z6

— NBC News (@NBCNews) April 10, 2018

The Queen and Attenborough discuss a conservation project to create a network of native forests across all 53 Commonwealth countries – the subject of the new documentary – and muse about how there might be many new trees in the Buckingham Palace garden in 50 years.

“Might easily be, yes,” the Queen says. Then, after a pause, she adds with a smile: “I won’t be here though.”

Attenborough chuckles but does not dwell on the remark, instead commenting on a nearby sundial positioned in the shade of a tree, rendering it useless.

“Isn’t it good, yes,” the Queen says, before flashing a wide grin at a person standing off camera. “Had we thought of that, that it was planted in the shade?” she asks with a chuckle, adding: “It wasn’t in the shade originally, I’m sure.”

The Queen then suggests the sundial could be moved, to which Attenborough responds that it depends whether she wishes to know the time or not.

The documentary, The Queen’s Green Planet, also features other members of the royal family as well as the actress and campaigner Angelina Jolie, who has taken part in the environmental project and is shown describing the Queen as “a lovely lady”.

The Queen and Attenborough have known each other for many years as the broadcaster, who has been making nature programs since the 1950s and continues to draw millions of viewers with every new one, has received a series of royal honours.

The two nonagenarians appear relaxed together, with little of the formality and rigid deference often seen in people addressing the monarch.

The pair look at trees planted in the palace garden to mark the birth of the Queen’s children, and Attenborough is seen squatting down to read the plaques as they try and work out which tree corresponds to which prince or princess.

The Queen, who routinely receives gifts from visitors from every corner of the planet, comments that the garden has benefitted from many gifts of roses and other plants.

“You know, I’ve been quite difficult to give presents to,” she says, drawing laughter from Attenborough.

“Of course they’ve said ‘Oh well, let’s give her a plant … or a tree or something.’ Which is very nice to have.”

-with AAP