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Despite the unscheduled stop the royal couple transferred to cars and continued on to the market town of Hay-on-Wye by road.
They finally arrived close to three hours late but emerged looking relaxed and smiling to be greeted by a crowd of several hundred well-wishers.
The Prince of Wales and the Duchess of Cornwall had a "hairy experience" when their royal helicopter was forced to make an emergency landing after it developed a technical fault.
Charles and Camilla were flying to Wales for a day-long tour to Hay and its popular annual festival of literature.
But minutes after boarding their flight they were urgently diverted to Denham Aerodrome, in Buckinghamshire.
A spokesman for the royal couple said: "The pilot carried out a controlled emergency landing after diverting to the airport."
"It was quite a hairy experience," a source close to the palace added.