A helicopter carrying the Duchess of Cornwall was involved in two near misses, reports reveal.
The Sikorsky S76 from the Royal Flight was taking the Duchess back to her home in Gloucestershire after last year's Sandringham Flower Show when both incidents occurred.
It came within 200ft of a De Havilland aircraft carrying parachutists, which had just taken off from an airfield at Chatteris, as it flew a prearranged flight plan south of Wisbech at around 12.20pm on July 25.
Prince Charles was not on board, as he was staying in Norfolk after attending the show with his wife.
A report by the UK Airprox Board says an air traffic controller at RAF Marham "perceived the severity of the incident as high". The Royal pilot described the risk as "medium".
Palace officials declined to comment. But one source said: "No-one was in any danger."
The report says protected low level corridors are declared when the Royals are flying.
It adds: "Pilots of civil aircraft operating near the published route should keep a good lookout and maintain adequate separation from the Royal helicopter."
The pilot of the De Havilland should have contacted air traffic controllers at RAF Lakenheath as he took off, but the report says he did not do so and they made "numerous attempts" to contact him when they realised the aircraft were heading towards each other.
A controller at RAF Marham said to a counterpart at Lakenheath: "Okay if you can keep trying to call him because he's now about to burst the bubble, he's within five miles and 3,000ft unless coordinated."
The De Havilland pilot responded after receiving a "short term conflict alert". Both aircraft took evasive action.
Airprox board members concluded that the regulations governing separation zones around Royal flights "introduced more ambiguity and complexity than clarity". They are recommending the rules are reviewed by the Civil Aviation Authority (CAA).
During the same flight, the Royal helicopter had to take evasive action to avoid a glider as it neared the Duchess's home at Highgrove House.
An airpox report says the Sikorsky pilot assessed the risk of collision as medium. The glider pilot could not be traced.
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