Investigators are looking into whether the last flying Vulcan bomber performed an illegal barrel roll weeks before its final flight.
A series of still images claim to show the Cold War-era aircraft performing the manoeuvre.
The Civil Aviation Authority (CAA) confirmed it was investigating whether the XH558 aircraft actually did perform the roll during a recent flight.
- Images provided by BDP Aviation
The Vulcan, based at Robin Hood Airport in Doncaster, flew for the final time last week.
A CAA spokesman said: "The CAA is investigating a recent flight when the Vulcan aircraft may have performed a roll manoeuvre. This did not occur during an air display.
"Although not normally allowed under its current permissions to fly, a roll is a benign manoeuvre and the Vulcan's maintenance support organisation has confirmed that the aircraft is safe to fly."
According to reports, the images of the Vulcan apparently performing the roll was taken near Grantham, Lincolnshire, on October 4.
The Vulcan, built in 1960, was prohibited from performing a roll under its long-standing permission to fly.
Owned by the Vulcan To The Sky Trust, the aircraft returned to the skies eight years ago, but it lost its permit to fly at the end of October after engineering firms that had supported its maintenance said they no longer had the skills to continue.
A spokesman for the trust said: "We can confirm that we are co-operating with the CAA to investigate allegations that one of our crews performed an aerobatic manoeuvre that is not allowed by our Permit to Fly.
"It would be inappropriate to comment further until key facts are established."
The Vulcan fleet was built to carry Britain's nuclear deterrent but its most famous operation - and the only time the aircraft dropped bombs in anger - was the 1982 attacks on Port Stanley airfield during the Falklands War.
The pilot at the controls of XH558 on its final flight, Martin Withers, led the first of those raids.