A celebrated children's writer dramatically changed his plea in mid-trial yesterday and admitted 11 indecent assaults on young girl fans during the height of his fame in the 1960s.

William Mayne, 76, was told by a judge that there was no alternative to a prison sentence for the deliberate targeting of the girls, all then under 15.

The jury at Teesside crown court was directed to return not guilty verdicts on two rape charges which the writer - described as "outstanding" in the Oxford Companion to Children's Literature - denied.

He was acquitted of two further charges of indecent assault.

Mayne, whose trilogy Earthfasts, Cradlefasts and Candlefasts, based on the legends of King Arthur, brought him a huge following in the 1960s, spent three days denying all the claims and dismissing them as make-believe. The first of his victims - now a woman in her fifties - went into the witness box to describe the shame and disgust which stopped her going to the police until last year.

The court heard that the offences took place over a 15-year period from 1960, when the Hull GP's son first received widespread literary acclaim. His career has continued to win plaudits for imaginative and independent-minded children's writing, and he was awarded the Guardian children's fiction prize in 1993.

The dark side to his life, involving the steady grooming of fans as young as eight at his homes in Ripon and Thornton Rust, both in North Yorkshire, did not emerge until 1999. The police did not proceed against him until other victims pressed the issue last year.

Mayne, now white-haired and balding, quietly answered "guilty" yesterday when the 11 charges were put to him again after an adjournment.

He was told by Judge David Bryant: "I must make it clear, though you remain on bail, I don't see any alternative to a custodial sentence in this case." He will be sentenced on April 30.

Mayne has published more than 60 books, the most recent last year, and written extensively for the BBC.

Richard Mansell, prosecuting, told the court: "He had something of an open-house arrangement where youngsters were allowed free run of his home."

Some of his victims stayed there and many were given rides in his 1928 British racing-green Bentley.

The court heard that the girls were "inspired and fascinated" by a highly talented man. They were also frightened and confused by his insistence that the abuse was something "girls really wanted" at their age.