The stag party for the prince's cousin was held on the Isle of Wight immediately after William received his pilot's wings, The Daily Mail reported. William also did flyovers of three royal houses in military Chinook helicopters and flew to a wedding.
The Royal Air Force has backed off its earlier claim that these flights were all legitimate parts of William's training.
"It was clear on occasions that those involved failed to appreciate how the media and public would perceive these flights and in retrospect there was a degree of naivety in the planning of these sorties," a spokesman for the Ministry of Defense said.
But the spokesman also said the prince's training period with the RAF had been valuable for everyone, giving him "valuable insight into the roles and capabilities of a modern Air Force." If William becomes king, he will be the nominal commander-in-chief of the British military.