Kate Middleton in November
Lawyers for Prince William
's longtime girlfriend Kate Middleton
have issued a final warning to the U.K. press: Stop photographing Middleton or face possible legal action.
Middleton's lawyers Harbottle & Lewis – who also represent William's father, Prince Charles – have resorted to secretly filming the paparazzi's pursuit of Middleton, 25, according to a letter sent to the Press Complaints Commission. The video footage would ostensibly support possible legal claims of harassment down the road.
"This letter is a final warning," a PCC spokesman tells PEOPLE. "It's an indication to all the editors that Miss Middleton's lawyers have evidence of harassment. Every newspaper editor will now receive a copy of that letter."
The royal family is said to be concerned for Middleton's safety, given the late Princess Diana's battles with the paparazzi throughout her life.
Middleton's lawyer Gerard Tyrrell wrote to the PCC: "As I have consistently made clear to both magazine and newspaper editors over the past nine months, my client strongly objects to having her photograph taken in a public place while going about her private business."
Of the film footage of the paparazzi, Tyrell wrote: "We have a large amount of very interesting but disturbing film footage.
The editors themselves cannot now be under any doubt whatsoever about the unlawful behaviour of the paparazzi from whom they buy photographs."
The increased focus on Middleton comes as speculation builds about when William, 24, might propose to his girlfriend of five years. "This is certainly not something they are going to rush into," a family friend told PEOPLE recently. "But their relationship is very strong."