Kensington Palace said she left in a car with William as he was driven to the Commonwealth Day service at Westminster Abbey in London.

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

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

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

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

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.

AFP's deputy news editor, Eric Baradat, told Sky News 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."

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 the King 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 of his diagnosis in early February.

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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 the Queen, 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.

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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".

Graham Smith, chief executive of Republic, said the controversy over the edited image will have "damaged their reputation significantly".

He told Sky News: "It goes to the heart of the issue that they are incredibly secretive."

He added that the royals "need to stop using the Commonwealth as a vehicle for their own PR".