Ampleforth, the UK's most famous Catholic school, is to become fully co-educational to prepare its pupils for modern university life.
The pound;18,000-a-year establishment, sited in rural North Yorkshire, has only 38 girls in the sixth form but hopes to increase that to 140 throughout the school within the next five years.
Actor Rupert Everett and politician Michael Ancram are old boys of the school, often referred to as the "Catholic Eton".
Headteacher Father Leo Chamberlain told The TES: "University is quite different from what it was when I was at Oxford. There is a complete mix of young women and men and I think that they need to learn to work together at an earlier stage, rather than just meeting at parties."
The expansion will be accompanied by a pound;5 million investment in boarding facilities. Ampleforth faces another major change with the announcement that Father Leo, who has been head since 1992, is to step down at the end of the year.
Father Leo, who has long been an outspoken and high-profile representative of independent schools, will become Master of St Benet's Hall, Oxford, which is owned by Ampleforth Abbey. Ampleforth's abbot, the Rev Timothy Wright, praised Father Leo for his "clear-sighted, principled and sometimes courageous participation in the debate over the future of education in this country".
He is a great defender of the independent sector's right to charitable status, saying education should be recognised as a "fundamental public benefit".
In a speech at the school last year, he said: "The independent schools, in providing near 50 per cent of the A grades at A-level each year in the harder subjects, are essential to the public weal. How wrong it is that universities are being virtually instructed to disadvantage independent school candidates."
Father Leo believes the model used by many private schools could be adopted by state schools. "What I would like to see, in broad terms, is for all schools to become trusts, with much more local management independence and much less centralised power," he said.
He stressed the importance of Catholic schools, whether maintained or independent, and takes issue with current proposals that he feels may threaten them. "It is important that heads retain the right to interview incoming candidates," he said.
Father Leo, who has been a monk-priest at Ampleforth for 40 years, will be succeeded by Father Gabriel Everitt, a former Anglican, who is currently the school's third master, a housemaster and head of Christian theology.