Prince William landed a Royal Air Force helicopter in the garden of his girlfriend Kate Middleton's family home but the Defence Ministry defended his actions, saying he achieved essential training objectives.
The News of the World reported on Sunday that 25-year-old William, second in line to the British throne, asked Middleton's parents' permission to land at their home in Berkshire, southern England, because of a shortage of landing slots in the neighbouring county Hampshire.
Aviation analyst and RAF-trained pilot Jon Lake told the weekly paper the April 3 flight was "ridiculous and inappropriate", but the ministry said the two-hour training mission was fully authorised as part of William's four-month RAF attachment.
"Battlefield helicopter crews routinely practise landing in fields and confined spaces away from their airfields as a vital part of their training for operations," it said in a statement.
"These highly honed skills are used daily in conflict zones such as Iraq and Afghanistan."
The ministry said helicopter bases continually sought permission from landowners to use their fields and there were only two fields permanently available in Hampshire.
"Opportunities to use alternatives are therefore regularly seized. 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."
William, the fourth successive generation of the monarchy to become an RAF pilot, received his Royal Air Force wings from his father Prince Charles on April 11 in a ceremony watched by Middleton.
Known as Flying Officer William Wales, he is due to start an attachment to the Royal Navy in June.
William broke up with Middleton, his long-time girlfriend, in April last year but they now appear to be together again, sparking feverish speculation among royal watchers about his marriage plans.