A monk is facing prison after admitting 20 counts of indecent assault on young boys at a preparatory school, which sends pupils to the country's leading Roman Catholic boarding college.

Father Piers Grant-Ferris, 71, a Benedictine monk at the Ampleforth monastic community, in North Yorkshire, was a teacher at Gilling Castle Preparatory School.

He was arrested and charged after a North Yorkshire Police inquiry into Ampleforth Abbey, which is attached to the country's leading Roman Catholic school, Ampleforth College.

Gilling Castle Prep School was used as a "feeder" school by the college, but it later merged with another school and is now known as St Martin's Ampleforth.

Yesterday, at Leeds Crown Court, Grant-Ferris admitted 20 counts of indecent assault on boys.

He denied six charges of indecent assault and one charge of gross indecency with a child.

The 15 complainants were aged under 12 at the time and the offences happened, between 1966 and 1975.

James Goss QC said the pleas were acceptable to the Crown and a trial in relation to the not guilty pleas was not necessary. The case was not opened.

The charges include fondling and slapping of the boys' buttocks, the court was told.

Judge Ian Dobkin said "all options" remained opened and he adjourned the case until the end of January for completion of pre-sentence reports.

He said: "All offences involving children, whatever the circumstances, are serious and the high probability is that prison will be considered in this case."

Patrick Cosgrove QC, mitigating, said his client understood the seriousness of the offences but said they were at the lower end of the scale.

Grant-Ferris, who lives at Ampleforth Abbey, was granted conditional bail.

An Ampleforth Abbey spokesman said Grant-Ferris's connection with the school ended in 1975.

Father Cuthbert Madden, Abbot of Ampleforth, said: "These were most regrettable offences involving a grievous abuse of trust.

"While they took place between 30 and 40 years ago, we offer a heartfelt apology for the hurt he caused to his victims, who were boys in our care at the time.

"When the incidents first came to light in 1975, he was immediately removed from further contact with students, and has since led an exemplary life of service as a valued assistant priest in several parishes for many years.

"A great deal has changed in the years since."

In September, Father Gregory Carroll was jailed after pleading guilty to 15 charges of indecent assault and five counts of gross indecency. The offences were all committed between 1973 and 1983 when the 66-year-old was a master at Junior House, Ampleforth College.

Frank Hopkinson, who was head of finance at the school for 40 years, was jailed for Internet porn, in October 2000.

The 836 pictures were all of men and young boys. They had been downloaded after Hopkinson left the college in 1989. In 1990, he was jailed for sexual offences against a 14-year-old boy.

In 1996, Father Bernard Green, who was a housemaster, admitted indecently assaulting a schoolboy the previous year.

Fr Madden said: "Today our schools have a framework of reporting, monitoring and pastoral supervision, which provide safeguards for child protection almost unrecognisable from those applying in those days.

''The measures we have in place conform to best practice and are regularly inspected by the Commission for Social Care Inspection."