​United Airlines is investigating a report that a giant rabbit died on one of its transatlantic flights.

Distraught breeder Annette Edwards from Worcestershire in central England told The Associated Press by phone Wednesday that Simon, a 10-month-old, 90-centimetre-long continental rabbit, had a vet check shortly before travelling from London's Heathrow Airport to Chicago's O'Hare airport.

She said Simon had been purchased by a celebrity she did not identify.

"Simon had his vet check just before getting on the plane," she said. "He was fit as a fiddle."

Edwards told the Sun newspaper that Simon was expected to grow to be the world's largest rabbit, surpassing his father Darius, who grew to 1.3 metres and more than 20 kilograms.

The airline said in a statement that it was "saddened" by the news, and had been in contact with Edwards and offered assistance.

The company is dealing with public relations issues after a passenger who would not give up his seat was dragged forcibly from a plane at the same airport three weeks ago.