Meghan, Duchess of Sussex, repeatedly held one or both of her hands in her pockets as she stepped out in a tuxedo-style dress with Prince Harry to visit Courtenay Creative, which runs programmes for young people looking to get into film.
But etiquette expert William Hanson warned the move was “unprofessional”.
Speaking to the Daily Mail, Mr Hanson said: “We know that Their Royal Highnesses The Duke and Duchess of Sussex are pioneering a new, relaxed, tactile branch of monarchy but having hands in the pocket is a step too far.
“There is good casual and there is bad casual. Placing hands in the pocket is too relaxed and unprofessional, however dressed down Prince Harry and his wife may have been.”
Mr Hanson described the incident as a “small blip” that could be “easily corrected”.
He said: “Will the world end? No. It is just a small blip in Meghan’s usually delightful manners that can be easily corrected for future engagements.”
The duchess, who is pregnant, showed off a hint of her baby bump in the short white dress by Maggie Marilyn which she teamed with a Gillian Anderson coat for the engagement.
Meghan and Harry were greeted by orcs from Lord of the Ring as they entered a room filled with musicians, actors, prosthetic experts and models for the outing in Wellington.
Meghan has been criticised for holding her hands in her pockets during a royal engagement
The duchess repeatedly held one or both of her hands in her pockets
The pose has been branded “unprofessional”
It comes as Harry and Meghan have become known for their relaxed approach to engagements in what is seen as a departure from royal tradition.
The romantic pair frequently put on public displays of affection.
And the couple are very casual with well-wishers, stopping to hug and pose for pictures with royal fans.
Harry and Meghan, who wed in May, are now in the final leg of their busy overseas tour which has also seen them visit Fiji, Tonga and Australia.
The duke and duchess will complete their final engagements on Wednesday, before they return to the UK the next day.