The Daily Mirror has apologised for publishing a photograph of Prince William's girlfriend, Kate Middleton.

She had made a formal complaint of harassment against the newspaper to the Press Complaints Commission (PCC).

Daily Mirror editor Richard Wallace said the paper "got it wrong and we sincerely regret that".

The PCC, which was contacted on Thursday after the publication of an image of Miss Middleton in the street, said it was still investigating.

Mr Wallace said: "The Daily Mirror has always steered a sensible course in the use of photographs of Kate Middleton and other public figures.

"On Thursday we published an innocuous picture of Ms Middleton walking down the street with a cup of coffee.

"It was taken by a freelance photographer in circumstances where we were later told she felt harassed.

"We got it wrong and we sincerely regret that."

The Mirror's photograph showed Miss Middleton, 25, walking to work, holding a cup and her car keys.

Miss Middleton had previously complained about the press intruding on her privacy.

In 2005, her lawyers contacted newspaper editors to ask the press to leave her and her family alone.

They claimed photographers had followed her almost every day and night since she left university.

In January, police had to escort the prince and Miss Middleton after they were surrounded by photographers while leaving a nightclub.

And she encountered a media scrum outside her flat on her birthday, also in January.

According to a PCC statement, she alleged that the photograph in the Mirror was "obtained as a result of harassment in breach of Clause 4 (Harassment) of the Code of Practice".

The clause states that "journalists must not engage in intimidation, harassment or persistent pursuit".

A spokesman said: "As far as the PCC is concerned, we're still in the process of investigating the complaint.

"It's for the complainant to see how she feels about this response."

News International - which publishes The Sun, News of the World, The Times and Sunday Times - has introduced a self-imposed ban on using paparazzi pictures of Miss Middleton.