The pound;9,234-a-year Magdalen College school, in Oxford, which was named Sunday Times school of the year in 2004, is helping to pay for the Oxford community school to win specialist status.
The move comes amid increasing pressure on private schools to offer benefits to the wider community, in line with the Government's proposed Charities Bill.
Writing in The TES today, Sir Cyril Taylor says: "Most independent schools provide a high standard of education as shown by their performance in public examinations," he says. "But in order to justify their charitable status they can and should help all our children receive a decent education."
His comments reiterate a warning he made yesterday at an independent school conference at Brighton college The 525-year-old Magdalen College is among a small set of leading private schools offering financial aid to neighbouring state schools.
The school contributed pound;8,000 towards the pound;50,000 needed by Oxford community school for it to be considered for specialist business and enterprise status. Other sponsors include the Sutton Trust.
The two schools are considering more links, including plans to send gifted state pupils to Magdalen for master classes and allowing private-school pupils to use the comprehensive school's large playing fields.
Steve Lund, head of Oxford community school, which will hear in the coming weeks whether its bid for specialist status has been successful, said:
"Oxford has a reputation for dreaming spires and elitism, but there are many deprived areas as well.
"It is important that we raise the aspirations of all children in this city. Both the state and private sectors have a lot to learn from each other."
Other private schools sponsoring local specialist schools include Harrow, King's College school, in Wimbledon, and Wakefield grammar. Dulwich College, in London, is proposing to sponsor a new city academy.