A helicopter carrying the Princess Royal was at the centre of a fire alert on Monday just before it was due to take off, a Fire and Rescue service said.
Princess Anne, 57, was on board the royal helicopter when emergency services were alerted in the afternoon.
The helicopter was about to leave Denbigh High School in North Wales - where she was using the grounds to board the aircraft - when the incident happened.
According to North Wales Fire and Rescue, the pilot used a carbon dioxide fire extinguisher on the helicopter.
Buckingham Palace said there was no fire, but there appeared to have been an electrical fault on board the craft.
The princess continued her visit - which had already taken in an engagement at a hospital - by car.
A fire brigade spokesman said: "North Wales Fire and Rescue Service were called to deal with reports of a helicopter on fire in the grounds of Denbigh High School at 1440 on Monday, October 8.
"Passengers were aboard the helicopter when the fire broke out. Passengers were evacuated from the aircraft, and the pilot used a carbon dioxide extinguisher to tackle the fire. On arrival, crews checked the helicopter to ensure the fire was fully extinguished.
"The fire appears to have been started by an electrical fault in the starter motor."
A spokesman for North Wales Police said: "As a result of what appeared to be a mechanical failure on the royal helicopter it was necessary to implement contingency plans to transport the princess from Denbigh to the other venue.
