The Duke and Duchess of Cambridge deserve "some privacy and some space" to bring up their children, David Cameron has said, after Kensington Palace accused paparazzi photographers of harassing Prince George.
The Prime Minister said he had "every sympathy" with William and Kate after it was claimed that some photographers were going to "extreme lengths" to get pictures of their two-year-old son.
In an unusually strongly worded open letter issued on Friday, Jason Knauf, the communications secretary of the Duke and Duchess, said the tactics were creating a "very real security risk".
Mr Cameron said that while the British press had behaved "very well" in not publishing paparazzi photographs of George, some foreign news outlets had to be persuaded not to use them.
He told BBC Breakfast: "I am concerned and I have every sympathy with the royal couple. They very generously have made available pictures of their wonderful children to the British press and the British press have behaved, frankly, very well because they printed those pictures.
"But they haven't been, as I understand it, printing paparazzi pictures and it's very important they don't because if you start printing the paparazzi pictures, the paparazzi go further and further in trying to get pictures of the royal family.
"What we need to do now is to persuade some of these foreign publications not to use these pictures. I do think William and Kate deserve some privacy and some space as they bring up their children.
"They do make available some pictures so we can all see and share in the joy they've got with their children but they should not be harassed by paparazzi."
The palace's hard-hitting letter detailed a series of incidents involving paparazzi, such as using children to draw George into view in playgrounds, and an incident last week when a photographer set up a "hide" in his car with sheets and supplies of food and drink as he staked out a play area.
The actions of photographers had left William and Kate "concerned about their ability to provide a childhood for Prince George and Princess Charlotte that is free from harassment and surveillance".
Other incidents included:
:: photographers pursuing cars leaving family homes
:: surveillance of the Berkshire home of the Duchess's parents, Carole and Michael Middleton
:: hiding in dunes to take photos of Prince George playing with his grandmother
:: hiding on private property in fields and woodland around the Duke and Duchess's home in Norfolk
In the letter, Mr Knauf said: "It is of course upsetting that such tactics - reminiscent as they are of past surveillance by groups intent on doing more than capturing images - are being deployed to profit from the image of a two-year-old boy.
"In a heightened security environment, such tactics are a risk to all involved.
"The worry is that it will not always be possible to quickly distinguish between someone taking photos and someone intending to do more immediate harm."
Scotland Yard warned that covert photographers faced putting themselves in danger at a time when the national security threat level from international terrorism is severe.
A statement said: "The covert actions of photographers have at times caused concerns during police protection operations when they have been considered a possible security threat."