Reuters/London

Britain's Prince William has warned two photographers against trailing his baby son George during walks in London parks with his nanny, saying their behaviour amounted to "harassment".

Lawyers acting for William, second-in-line to the throne, and his pregnant wife Kate, have written to the freelance photographers asking them to stop following George, William's office said.

It came after an incident in a central London park last week when one individual was spotted near George and his nanny.

"They are obviously concerned," a spokesman for the royal couple said. He added the men had "over a period of time established a pattern of behaviour that we think amounts to harassment of Prince George and of his nanny".

"It's more the fundamental weirdness of two grown men following a 14-month-old baby around London that we're concerned about," the spokesman added.

The issue of harassment by the paparazzi is particularly sensitive for William whose mother Princess Diana was killed with her lover Dodi al-Fayed in Paris in 1997 when her limousine crashed into the wall of a tunnel as they tried to flee a posse of chasing photographers.

He has been determined to protect his family from the press.

George was born in July last year and has spent most of his life since then out of the spotlight, with only a handful of public appearances.

"They want Prince George to lead as ordinary a life as possible as a child and he obviously can't do that if people are going to parks where children play and taking pictures of him," the spokesman said.

He said they believed some pictures of George had appeared in European publications but not in Britain, where the press has exercised greater restraint since Diana's death.

However, the royals complained about harassment of Kate before the couple married in 2011. A year after their wedding, European magazines printed topless pictures of Kate taken while she was on holiday in France.