Prince William was awarded his RAF wings on 11 April
Defence officials have justified giving Prince William permission to land a helicopter in a field belonging to his girlfriend Kate Middleton's family.
The Ministry of Defence (MoD) issued a statement after the News of the World reported William touched down at the Middletons' Berkshire home on 3 April.
Officials said the sortie had been fully authorised as part of the prince's attachment to the RAF.
The Chinook landed for 20 seconds, but no-one got on or off, the MoD said.
The News of the World said the 25-year-old prince touched down on land at the Middletons' home in Bucklebury, Berkshire, during a 100-mile, two-hour training exercise.
He flew out of RAF Odiham, Hampshire, but, because of a shortage of landing spots in the county, asked for permission to use the Middletons' land, the paper claimed.
Kate and her parents, Michael and Carole, were watching from their home as he landed, it also said.
The flight was described as "ridiculous and inappropriate" by aviation author Jon Lake, quoted in the News of the World.
"This is an absolute waste of training hours on the Chinook helicopter that the military are hard-pressed to afford," he told the paper.
"No other pilot at Prince William's stage of training would be allowed anywhere near the left-hand seat of a Chinook."
In a statement, the MoD said battlefield helicopter crews routinely practised landing in fields and confined spaces away from airfields as part of their training for conflicts such as Iraq and Afghanistan.
"Helicopter bases continually seek permission from land owners to use their fields and there are only two fields permanently available in Hampshire. Opportunities to use alternatives are therefore regularly seized."
It added: "The aircraft landed in the field, after taking all necessary safety precautions, and was on the ground for 20 seconds.
"No-one got on or off the aircraft. This was very much a routine training sortie that achieved essential training objectives."
Prince William was awarded his RAF pilot's wings on 11 April after training with the air force since January.
His shortened training course was designed to make him a competent flier, but an RAF pilot would usually spend up to four years under instruction.
A graduate of Sandhurst military academy, William will now undertake a similar attachment to the Royal Navy.
His training periods with the RAF and Navy are familiarisation attachments, aiming to prepare him for his future role at the head of the armed forces.