Prince William dropped his trousers during a 72-hour booze-fuelled stag do for his cousin Peter Phillips over the weekend.

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William got so drunk on the first night of the boys only trip to Britain’s Isle of Wight – which was held to celebrate Peter’s impending marriage to Autumn Kelly – he flashed his blue boxer shorts during a drinking game at his hotel in the early hours of Saturday morning (12.04.08).

Earlier in the evening, William’s brother Prince Harry got the party started when the 23-strong group – who boarded a ferry from Southampton to Cowes on Friday morning (11.04.08) – headed to the Anchor pub.

Local Sophie Beak told Britain’s The Sun newspaper: “I was sitting watching the band when Harry came over holding a funnel and a pint of beer and said I had to have a drink.

“He put the funnel in my mouth and began pouring lager down it – but after two gulps I had to give up. He laughed, gave me a big hug and told me to have a good night.”

At one point the night took a nasty turn when the royal brothers – who wore t-shirts with nicknames ‘Wills’ and ‘Hazza’ emblazoned on the back – feared they had lost the drunken stag.

Harry was heard exclaiming: “Where the f**k is the stag? We’ve lost the f***ing stag?”

Luckily, Peter – the son of Princess Anne – was found slumped in a shop doorway, and his friends escorted him back to their hotel where the party continued.

Despite having thumping hangovers, on Saturday morning the group managed to get up to go sailing round Cowes marina.

Peter, 30, was immediately forced to down a pint of beer mixed with vodka, before his friends drew a pair of glasses and a moustache on his face with a black marker pen.

In the evening, the group continued to torment the groom-to-be by making him wear a lycra catsuit and taping a pint glass to his hand as they embarked on another pub crawl.

Peter is due to wed Autumn at Windsor’s St. George’s Chapel, in Berkshire, on May 17.