Kate Middleton has made a formal complaint over alleged harassment to the Press Complaints Commission after the publication of a picture of her in yesterday’s Daily Mirror.

After relying on private assurances from editors, lawyers for Ms Middleton have stepped up their campaign to protect her from a daily pursuit by paparazzi. Ms Middleton, who is the girlfriend of Prince William, objected to a picture that showed her clutching a take-away coffee cup and car keys as she headed to work.

The accompanying article suggested that a “stoney-faced” Ms Middleton was likely to be unhappy after the Prince had been photographed with other women during a night out in Bournemouth.

Lawyers for Ms Middleton, 25, are believed to be preparing a test case to protect her privacy and to stop what they regard as dangerous harassment by photographers.

Sir Christopher Meyer, chairman of the Press Complaints Commission (PCC), made an informal appeal to newspaper editors to “back off” Ms Middleton this year.

However, Harbottle & Lewis, solicitor for both Ms Middleton and the Prince of Wales, warned the media that any further breach of her privacy would result in an official complaint.

Gerrard Tyrrell, her solicitor, said: “My client strongly objects to having her photograph taken in a public place while going about her private business.”

The Mirror picture triggered an immediate complaint that the paper was guilty of a breach of Clause 4 (Harassment) of the PCC Code of Practice.

The commission said in a statement: “The complaint will now be investigated in the normal way and the commission will not be making any further statement on the subject until the matter has been dealt with.” Harbottle & Lewis claims to have “disturbing” film of photographers intruding on Miss Middleton’s privacy and potentially jeopardising her safety.

Paparazzi activity outside her London home reached a frenzy during her birthday in January. The PCC delivered a “final warning” to editors, reminding them that Ms Middleton remained a private individual. News International, owner of The Times, does not use paparazzi pictures of her.

If the commission’s complaint is rejected, Miss Middleton’s lawyers could use the Protection from Harassment Act 1997 against individuals.

It is her first official complaint to the commission.

Royal aides revealed that Prince William wanted “more than anything” for photographers to stop harassing his long-term girlfriend. His concerns over the media come after the death of his mother, Diana, Princess of Wales, who was being pursued by paparazzi when she was killed in a car accident in Paris in 1997.

In 1996 the Princess, with the Duchess of York, reported the Daily Mirrorto the commission over photographs of them on holiday with their children in France. She later dropped the case.

Three years earlier the same paper had published pictures of the Princess in a gym, wearing a leotard and using an exercise machine.

The pictures, taken with a hidden camera, were dubbed the “peeping Tom” photographs. She launched legal action and Bryce Taylor, the former owner of the gym, and Mirror Group Newspapers settled out of court.

In 1996 the Princess obtained a court injunction against Martin Stenning, a photographer, which banned him from going within 300 yards of her.

Clarence House last night refused to discuss Ms Middleton’s case.

A spokesman for the Mirror said: “We will provide the necessary thoughts and comments to the PCC. We will not be commenting any further at this stage.”

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