The picture, sent to a teenage girl suffering from terminal leukaemia in the 1960s, went under the hammer in Aylsham, Norfolk.

Andrew Bullock, of auctioneers Keys, said it was thought Harrison had copied the signatures of his fellow band members - Paul McCartney, Ringo Starr and John Lennon.

"We've sent it to a handwriting expert and it's just about 100% certain that they were done by George," he said.