The Duke and Duchess of Cambridge were on a passenger jet that narrowly avoided ploughing into another plane, it's been claimed.

Prince William and Kate Middleton were flying home to London on November 2 after visiting Denmark on a humanitarian mission.

The Duke and Duchess of Cambridge in Copenhagen, with the Crown Princess Mary and Crown Prince Frederik of Denmark. Photos: Getty

The British Airways A320 carrying the royal couple and other passengers was about to land at Heathrow when the aircraft made a steep climb at the last minute to avoid a plane that hadn't cleared a runway in time, UK newspaper Daily Mail reports.

A BA spokesperson has confirmed its crew was forced to perform the maneuver but said it was "standard procedure".

"Our crew were told by air traffic control to perform a go-around – a perfectly standard operation with no safety implications – because an aircraft from another airline hadn't fully cleared the runway.

"Our aircraft landed safely and customers left the plane as normal."

But The People newspaper quotes a source saying the Duchess was "deeply unsettled" by the episode.

"This really was a serious incident and would have been terrifying for those on board.

"For anyone who didn't know what was going on it would have been extremely worrying."

Clarence House and Heathrow Airport have refused to comment on the incident.

The Duke and Duchess of Cambridge in Copenhagen in November.