Wearing a floral dress and no hat, Queen Elizabeth strolls in dappled sunlight through the garden at Buckingham Palace with an old acquaintance, making quips and even mentioning the prospect of her own demise in a light-hearted way.

The scene, filmed last summer for a documentary broadcast in Britain last week, offers an unusually informal glimpse of the 92-year-old monarch, who has given very little away of her inner self during her 66-year reign.

She is seen walking and chatting with David Attenborough, 91, a much-loved naturalist and broadcaster known for lavish series like Planet Earth and Blue Planet.

The pair 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 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, then 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 on 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”.

In a separate excerpt from the queen’s walk with Attenborough, released by ITV ahead of the broadcast, the quiet of the palace garden is broken by the noise of a helicopter hovering overhead.

“Why do they always go round and round when you want to talk?” the queen asks, adding: “Sounds like President Trump, or President Obama”, apparently referring to the aircraft used by the US presidents during visits to her.

During another light-hearted moment, the monarch spots a sapling bent over at an alarming angle, remarking: ”Somebody sat on it I think, at the garden party.”

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

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, so,” 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.” – Reuters/Estelle Shirbon