The Duke of Cambridge was relieved of nappy-changing duties by Kate Middleton so he could go on a boozy weekend with friends.
Prince George was born nearly three weeks ago and now Prince William is keen to catch up with his friends.
The second in line to the throne caught the train from London Paddington to Exeter St Davids with 15 of his closest friends.
William's party had a private carriage marked X. A female passenger noticed the rowdy behaviour. "I walked through the carriage on the way to the buffet car and spotted William with a bunch of chaps swigging beer like it was going out of fashion," she told The Sun.
"The tables were covered with cans and miniature bottles of gin and one chap seemed to be wearing gold lamé shorts."
Other people were unaware of the presence of royals on the train. "There was an older man next to me who had no idea William was sitting just yards away. He nodded at them and said, 'There's a right bunch of lads in there,'" added the passenger.
The 31-year-old Prince, who was dressed casually in chinos and shirt, was with one of his best friends, Guy Pelly.
The London nightclub impresario was a potential godfather for Prince William and Kate's son. However, it's claimed that William's choice was vetoed by the Queen.
"William and Kate's newborn may have up to eight godparents - however, all submissions MUST be approved by The Queen herself," a source told the National Enquirer.
"He wasn't fit to be one of her great-grandchild's godparents - not a man who's been dubbed 'dubious' by the British press and has had numerous run-ins with the law, including a DUI arrest."
William returned to his duties as a rescue pilot with RAF recently after two weeks' paternity leave.
Meanwhile, the Duchess of Cambridge remains with Prince George at her parents' £4.85million mansion in Bucklebury, Berkshire.