The group, including William's close friend Guy Pelly, were travelling in a private carriage from London Paddington to Exeter St Davids yesterday.
A fellow passenger told The Sun: "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.
Quite a handful: William has finished his paternity leave and is back at work with the RAF
PA
"The tables were covered with cans and miniature bottles of gin and one chap seemed to be wearing gold lamé shorts."
William - who was wearing chinos and a smart shirt - returned from paternity leave last week and reported back to work with the Royal Air Force.
Astonishingly, it seems not all passengers realised who he was.
The insider added: "There was an older man next to me who had no idea William was sitting just yards away.
"He nodded towards them and said, 'There's a right bunch of lads in there.' "
