Ryanair's 'verification flight' over Scotland on 24 May lacked the necessary equipment to determine whether high levels of volcanic ash were present, according to the UK Civil Aviation Authority (CAA).
The Ireland-based budget carrier operated one of its Boeing 737-800s on a large circuit of Scotland and said that a post-flight inspection of the airframe and engine had found no evidence of ash.
It called for the resumption of flights after several airlines cancelled services to and from Scotland following predictions of high concentrations of airborne ash.
"Our understanding from looking at the radar readings is that the Ryanair flight did not actually enter the notified area of high concentrations of ash," said the CAA.
The ash was not always visible, she added, and it was not uniformly spread through different flight levels. High, medium and low concentrations of ash had all been present over Scotland at the time of the Ryanair flight.
The other difficulty with the flight, she said, was that the Ryanair aircraft had been uninstrumented, "so it would be difficult for them to know what levels of ash they were flying through".
With no means of recording the density of ash they had encountered "there's no evidence that what they are saying is accurate".