The school, which educates the Prime Minister's two sons Euan, 15, and Nicky, 13, has written to parents asking for a "voluntary" monthly covenant of pounds 30 for a pupil at The Oratory and pounds 15 per additional child.
For the Blairs, whose combined annual income is said to be above pounds 300,000, pounds 45 would not pose too much of a financial burden but the prospect of a state school asking parents for regular payments is likely to ignite a political row.
The letter announcing the levy was reportedly sent to parents yesterday. John McIntosh, the head, and the chairman of the governors Father Ignatius Harrison, also the Provost of the Oratory, blamed changes in the way the Government funds school which has led to a budget deficit for the current financial year of pounds 250,000. They warned that the deficit put the school's recent achievements "in jeopardy".
It is not unusual for schools, particularly church schools, to ask parents for donations. But the size of the request from the Oratory is believed to be the largest of its kind from a state school.
There was criticism from some parents last night that "voluntary" amounted to little more than "moral blackmail".
The Conservatives spokeswoman on education, Theresa May, said: "These staggering revelations show that the Government is intent on attacking excellence in education. Tony Blair has exposed grammar schools to the threat of closure and has abolished the opt-out status for grant-maintained schools meaning their budget is channelled through local authorities. This has left the London Oratory with such a shortfall that it is asking parents for hand-out. The bitter irony is that Tony Blair may now be personally having to pay for his own mistakes."
Downing Street refused to be drawn on whether Mr Blair would pay the levy. A spokesman said: "We have always said the education of the children is a private matter."Reuse content