Private schools should be forced to team up with those from the state sector or face losing their charitable status, the Government was urged today.

Ministers should also increase public spending on partnerships between fee-paying and state schools to £100 million from the £4 million they have invested over the last five years.

The proposals were put forward by independent Brighton College headmaster Anthony Seldon and Antony Edkins, head of state sector Falmer High School in Brighton in a pamphlet for centre-left think tank the Institute for Public Policy Research.

The incoming Labour Government set up the independent/state school partnerships initiative in November 1997 and 200 schools and 19,000 pupils in England had been involved since then, they said.

But the £4 million the Government has spent during that period was less than one year's budget of a large comprehensive school.

The heads argued that no other country allowed such a "gulf" to exist between two education systems.

"The divide is deeply damaging in a variety of ways, social, pastoral and academic."

However, abolishing private schools would be legally and politically impossible as well as "morally highly questionable" and so partnership was the only way forward, they argued. As there were far fewer independent schools, they would all have to be compelled to forge links with state primaries and secondaries, said the heads.

The fact that in state secondaries the average class size was 17.1, compared with 9.9 in public schools, was the key to why the "learning experience" in the two sectors was so different.

While state sector spending per pupil was rising and was set to continue to do so, investment was growing at an even faster rate in fee-paying schools, which earlier this year passed the half-million pupil mark for the first time.

School inspectors should treat partnership as "mandatory", as happened in the case of specialist comprehensives, which can lose that status if they do not work with others in their area, the heads added.

A recent Cabinet Office report said a private school's charitable status should depend on it opening up its facilities to state neighbours.

But the heads urged the Government to make partnerships compulsory even though "Labour might find it politically too awkward to put the squeeze on independent schools by denying them charitable status if they do not engage in partnership".

They also proposed making it compulsory for all new teachers to spend their first three years teaching in state schools, before being allowed to switch sectors.

And private schools should use some of the money they saved through having charitable status on full-time "partnership co-ordinators" to deal with the planning and administration of links.

The heads said there were many difficulties involved in expanding links between the two sectors, not least of which was state school "teacher resistance and suspicion".

And they disclosed that independent school heads had received complaints from parents who objected to money being spent on state school pupils "with no immediate benefit to their own children".

The authors said the state schools and their pupils did get the most out of partnerships and it would be "foolish" not to admit that fact.

But, while state school pupils stood to gain the most, children from both schools would learn about their peers from different backgrounds.

The pamphlet was published on the day that Brighton College, where boarding fees are £16,000 a year, hosted a one-day conference in state-private school partnerships.

School standards minister David Miliband was among those due to address it, along with IPPR education researcher Joe Hallgarten.

At Mr Edkins's school, more than 40 per cent of children qualify for free meals, the standard poverty yardstick and average family income, at £11,000 a year, is the same as Brighton College's day fees.