David Attenborough met with the Cambridges at a private viewing of his new environmental documentary, A Life On Our Planet this week.
The veteran broadcaster - who is in the at risk Covid group - also gave Prince George a memento to mark their meeting at Kensington Palace, a fossilised giant shark’s tooth.
On Sunday, Kensington Royal Instagram posted pictures of the outing, but social media users pointed out the family may have broken the rule of 6 guidelines.
With six in the picture - not wearing masks - the was also at least one photographer and security guard in attendance.
One user said: "David and the Royals equal six on their own, taking into consideration security and the photographer… were government guidelines ignored?"
A second said: "Sir David Attenborough is an amazingly fantastic guy. However can someone please clarify all the exemptions to the rule of only six.
"There are only six of them in the garden however, who took the photo please?"
"Erm… am I missing something here? One rule for them another for us," a third added.
In a separate Tweet, users commented on the fact Prince William, Kate and David Attenborough are not wearing masks in the photographs.
One said: "Are they supposed to be wearing masks?
"I am confused, earlier today a family was shamed for not wearing masks while getting into their car.
"Here a family is hanging out with a man in the target age group to be susceptible to Covid with no masks. Which one is it?"
The Palace have been approached for comment.
The pictures were taken earlier this week after the 94-year-old broadcaster joined the Cambridges to watch an outdoor screening of his upcoming film.
The film, "David Attenborough: A Life On Our Planet," sets out his "witness statement" on the destruction of the environment and ideas on how humans can still put it right.
Prince William and Prince Charles have stepped in to fight against climate change since Prince Harry stepped down as a senior member of the Royal Family.
Attenborough and William are working together on William's Earthshot Prize, an ambitious global environment project announced last December to combat climate issues.
Details about the Earthshot Prize are expected in the coming weeks, as the men continue to support each other in their mission to tackle some of the biggest environmental challenges the planet faces.
David Attenborough: A Life On Our Planet, produced by WWF and Silverback Films, premieres in cinemas on Monday and will launch on Netflix on October 4.