Police found the man lying down in the boot of the vehicle attempting to shoot photos with a long lens through a small gap in his "hide".
Royal insiders say the Duke in particular is desperate that “history should not repeat itself” and wants to protect both Prince George and Princess Charlotte from the kind of intrusion that his mother Diana, Princess of Wales was subjected to.
The Duke has always believed paparazzi photographers in Paris were ultimately responsible for his mother’s death in 1997, when her car crashed at speed in an underpass following a pursuit by photographers on mopeds.
The letter, sent to 24 worldwide media industry watchdog bodies, says it is clear that Prince George has become the "number one target" in the Royal Family for unscrupulous freelance photographers who sell their images abroad.
It states: "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.”
It highlights the Duke and Duchess of Cambridge's desire for their two young children not to have to "grow up exclusively behind palace gates and in walled gardens" and says they want them to be free to play in public and semi-public spaces with others without being photographed.
The letter, signed by Jason Knauf, the Cambridges' communications secretary, states that such tactics have left the royal couple "concerned" about their ability to provide a harassment-free childhood.
It says: "They know that almost all parents love to share photos of their children and they themselves enjoy doing so.
"But they know every parent would object to anyone – particularly strangers – taking photos of their children without their permission. Every parent would understand their deep unease at only learning they had been followed and watched days later when photographs emerged."
The letter thanks all British media organisations, and most of those in the Commonwealth and the US, for operating a policy of refusing to publish unauthorised photographs but said a "handful" of international organisations were still willing to pay.
It notes that such pictures are "usually dressed up with fun, positive language about the 'cute', 'adorable' photos and happy write ups about the family" and give no hint to the underhand tactics used to procure them.
In May, Kensington Palace warned the media against harassing the family at their home, Anmer Hall on the Sandringham estate in Norfolk.
Letters stated that while members of the Royal Family and their guests are in residence they have "a more than reasonable expectation of privacy".
And last October, the couple started legal action against two photographers they accused of carrying out "surveillance" on Prince George.
Although the Duke and Duchess are prepared to take legal action, many foreign publications are now offering anonymity to the photographers to make it more difficult to pursue them.
Taking action against breaches of privacy in foreign courts can also be a lengthy and costly business.
A privacy case in the French courts which the Duke and Duchess took against a photographer who took topless pictures of the Duchess at a French villa during a holiday in 2012 is still rumbling on almost three years later.