Time was when a potential Royal in-law would have behaved with a certain dignity and decorum.

But the modern breed of well-connected youngster has little time for such etiquette – as shown by this embarrassing picture of Chelsy Davy’s brother Shaun sharing a pair of underpants with his girlfriend.

It is just one of several images of Prince Harry’s possible future brother-in-law that have been posted on the same public website that recently showed James Middleton – the brother of Prince William’s girlfriend Kate – in similarly compromising positions.

Briefs Encounter: Chelsy Davy's brother Shaun and girlfriend Anna share a pair of pants

One picture shows Shaun smoking a rolled-up cigarette that looks suspiciously like a cannabis joint. But in a statement released through the Prince of Wales’s solicitors Harbottle & Lewis, Shaun ‘completely denies’ that the cigarette contains the illegal drug.

Another photograph shows Shaun with a knotted handkerchief on his head, as an unseen friend prepares to insert what looks like another couple of cigarettes up each of Shaun’s nostrils.

Shaun, 24, and his girlfriend Anna Barnard, a Bristol University graduate, are members of Prince Harry’s social circle.

A source told The Mail on Sunday: ‘Chelsy, Shaun and Anna have been on a number of trips together in the past two years.

‘They include visits to New York, Cuba, South Africa, Brazil and Mexico. They have had a lot of fun getting drunk and seeing the world.

‘Prince Harry has been on a few of the trips and Shaun tagged along on a boys’ safari in Botswana. Harry has been on trips that include late-night binges in which large amounts of alcohol were consumed.’

Sean and Chelsy Davy joined Prince Harry and Prince William at the Concert For Diana in July

The publication of the images on theinternetforum.com will not help 23-year-old Harry’s attempts to nurture a more dutiful image. He has  vowed to tame his party lifestyle and concentrate on his military career.

A source said: ‘Shaun happily posed for the pictures but he obviously doesn’t want to cause problems for his sister. The Princes and their girlfriends are told time and time again to be careful with what they do in public, and to be extra careful when being photographed to make sure there is nothing that can be potentially damaging.

‘Shaun is aware of this but he’s a party person, just like his sister, and doesn’t feel as if he has to watch every move he makes.’

But, inevitably, photographs of anyone linked to the Royals engaging in boisterous behaviour finds its way on to the internet.

The pictures of Kate Middleton’s brother James showed him dressed as a French maid, and last week we revealed how images of 22-year-old Chelsy at a chav fancy-dress party had also been posted on the web.