Queen Elizabeth was just caught making a joke about President Trump on camera.

Whoever said royals can’t have fun has clearly never taken Queen Elizabeth II on a relaxing stroll through her private garden.

The 91-year-old monarch has been busted making a quip about the current US President, Donald Trump, while being interviewed by the legendary Sir David Attenborough for a TV special set to air next week.

As the pair – who were born just three weeks apart in 1926 – wander around the gardens of Buckingham Palace, a loud aircraft can be heard overhead.

“Why do they always go round and round when you want to talk?” Her Majesty remarks, before adding, “Sounds like President Trump … or President Obama.”

Watch: The Queen takes a cheeky jab at President Trump in a new TV interview.

The joke forms part of a television special – The Queen’s Green Planet – which also sees the world’s longest-reigning monarch light-heartedly reflect upon her own mortality.

“I suppose actually the trees with which you will be presented are going to change as our climate changes and that there will be all kinds of different trees growing here in another 50 years maybe,” Attenborough, known for hosting documentary series such as Planet Earth and Blue Planet, muses as they stroll through the garden.

“It might easily be, yes…. I won’t be here though,” the Queen smiles in reply.

The pair also joke about the curious decision to place a sundial in the shade of the garden, rendering it completely useless.

"Had we thought of that, that it was planted in the shade?" she asks a person off camera 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 Queen and Attenborough have known each other for a long time, with the broadcaster receiving several royal honours over the years.

The two nonagenarians appear relaxed together in the TV special, which focuses on a conservation project to create a network of native forests across all 53 Commonwealth countries.

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.

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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."

The documentary 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".

With AAP.