The Prince of Wales’s lawyers are preparing a test case to protect the privacy of Kate Middleton and stop the relentess harassment of her by the paparazzi.

It is understood that the lawyers are investigating criminal and civil law on harassment to curb the activities of certain photographers. They are also looking at using the Human Rights Act as well as building on recent privacy rulings to restrict the paparazzi.

The move comes in response to an increasingly frenetic level of attention paid to Ms Middleton, Prince William’s girlfriend, including photographers chasing her on their motorbikes and by car. Senior members of the Royal Household have likened it to the pursuit of Diana, Princess of Wales, that ended in her death.

Yesterday, as Ms Middleton left her home in Chelsea, southwest London, at 9am, there were 12 photographers and three television crews waiting outside. Prince William and Prince Charles are concerned that Ms Middleton is facing what amounts to “harassment and intrusion”.

A court case on behalf of Ms Middleton, who celebrates her 25th birthday today, would be a high-risk strategy for the Royal Family, even if they were only indirectly associated. But it could define the boundaries between legitimate press photography and an invasion of privacy.

The law firm Harbottle & Lewis is warning newspapers and broadcasters to “sharpen up their act” or face legal action. A spokesman for Clarence House said: “Ms Middleton is a private individual and as such can expect to have the privacy and private life that would be enjoyed by any member of the public.”

Prince William has been bolstered by a recent legal victory for his father, in which a court ruled that The Mail on Sunday had infringed the Prince of Wales’s copyright and confidentiality by publishing extracts from private diaries about the handover of Hong Kong.

Harbottle & Lewis is understood to be in continual contact with media organisations in an effort to secure an informal agreement to protect her. If that fails, lawyers have planned moves with three targets: the photographers themselves, media organisations here and abroad, and their regulators.

A first port of call would be a complaint to the Press Complaints Commission. Tim Toulmin, spokesman for the commission, said: “Since the press complaints code was rewritten after the death of Diana, we have dealt with thousands of cases where we call people off from harassing others — with success. We deal with these complaints informally and quickly and I cannot think of a case when it has not worked.”

At the same time individual photographers could be targeted through use of the Protection from Harassment Act 1997. The Act provides for criminal and civil proceedings and could be used against the worst offenders to warn off others.

Gill Phillips, media lawyer with Times Newspapers, said that this would have a salutary effect. “In the light of recent rulings that are all very much in favour of recognising people’s privacy rights, I think there’s no doubt that courts would be sympathetic to doing something here.”

However, Paul Sanders, the Times picture editor, said that it would be difficult to define the limits. “It is a fine line — generally it is held to be crossing the limits of what is acceptable if you go on to private property, or block the person, invade their personal space or try to provoke them in some way to get a response.” But the advent of mobile phones with high-quality cameras, and the fact that many photographers act as freelances selling to agents, made it hard to control taking pictures of people who were in the public eye.

The hand of Ms Middleton’s lawyers has been strengthened by a ruling from the European Court of Human Rights won by Princess Caroline of Monaco in 2004. The court held that photographs published in a magazine were a breach of her privacy.

Antony White, QC, a media barrister with Matrix Chambers, said that as well as the statutory law on harassment, lawyers for Ms Middleton could use the common law to obtain protection. The courts might build on the ruling won by Princess Caroline and grant a general injunction against photographers as a group without naming specific individuals.

Last night a senior editorial executive at The Sun said that the paper had decided not to use any more paparazzi pictures of Ms Middleton. They they would no longer be part of the “media circus” outside her door, he added.