Kensington Palace said the Princess of Wales left in a car with the Prince of Wales as William was driven to the Commonwealth Day service at Westminster Abbey in London.

Kate did not attend the service, but is understood to have a private appointment.

The Princess of Wales could be seen looking out of the window.

Earlier, she apologised "for any confusion" after admitting to "editing" a controversial Mother's Day image of her with her three children.

In a statement, Kate, 42, said: "Like many amateur photographers, I do occasionally experiment with editing.

"I wanted to express my apologies for any confusion the family photograph we shared yesterday caused.

"I hope everyone celebrating had a very happy Mother's Day. C."

The image was circulated by a number of picture agencies on Sunday before Reuters, Associated Press (AP), Getty Images and Agence France-Presse (AFP) told media outlets to "kill" the photo from their systems and archives.

AP told Sky News the photo showed an "inconsistency in the alignment of Princess Charlotte's left hand".

A Reuters spokesperson said: "The altered photo didn't meet Reuters standards of image quality" hence the withdrawal.

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AFP's deputy news editor, Eric Baradat, said there was "very strange business" surrounding the photo, especially when it was enlarged.

Mr Baradat added: "It becomes obvious that it's been manipulated, altered, doctored... really badly. So in a way that is really amateur."

Read more: What led to palace admission over 'manipulated' photo of Kate

The picture was released to celebrate Mother's Day.

It comes following speculation surrounding Kate's health since she spent a short period in hospital in January for abdominal surgery, shortly after her 42nd birthday.

Kensington Palace later confirmed the surgery was successful and she would be taking time to recover at home in Windsor.

King absent but 'deeply touched'

Commonwealth Day is celebrated yearly and the association marked its 75th anniversary on Monday - though King Charles was not in attendance as he continues to receive treatment for cancer.

The event is the most significant royal occasion the monarch has missed since he was forced to postpone all public-facing duties, since the announcement was made in early February.

In a video address, the 75-year-old highlighted how "diversity" was the Commonwealth's core "strength", and the union of 56 member countries was a "precious source of strength, inspiration, and pride" to him.

Charles said in the pre-recorded message: "In recent weeks, I have been most deeply touched by your wonderfully kind and thoughtful good wishes for my health and, in return, can only continue to serve you, to the best of my ability, throughout the Commonwealth.

"My belief in our shared endeavours and in the potential in our people remains as sure and strong as it has ever been. I have no doubt that we will continue to support one another across the Commonwealth, as together, we continue this vital journey."

Prince William and Queen Camilla, who wore a turquoise coat and hat, attended the service along with other royals including the Duke and Duchess of Edinburgh, Princess Anne, and the Duke of Kent.

The ceremony featured performances from Ugandan dance troupe The Ghetto Kids which made the royals smile with their traditional music and moves, Canadian pianist Spencer Klymyshyn, and a New Zealand soprano and baritone duo Isabella Moore and Benson Wilson.

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'Down with the Crown'

Meanwhile, outside Westminster Abbey anti-monarchy protesters from Republic held yellow placards reading "Down with the Crown" while also chanting loudly and banging drums.

Graham Smith, chief executive of Republic, denounced the royals saying they "need to stop using the Commonwealth as a vehicle for their own PR".

Mr Smith added: "The platitudes we hear from Charles and Camilla only serve to paper over serious concerns about human rights abuses and a lack of democracy in many Commonwealth countries."

He said the Royal Family gained from slavery over time and "presided over an empire" which "damaged people's lives".