When the prime minister gathered the heads of independent schools for a Downing Street summit last year, the message was clear: he wanted all private schools to sponsor academies.
But David Cameron's plan appears set to fail. The Headmasters' and Headmistresses' Conference (HMC) group of top independent schools has delivered a devastating blow to the ambition, warning that the risks involved in sponsoring primaries are too great.
David Levin, chairman of the HMC's primary school academy group, has revealed to TES that the large pension deficits for support staff - which private schools would have to take on if they sponsored a state primary school - were a "deal-breaker".
There are also huge concerns about the burden of managing the administration, finance and personnel of an entire academy, said Mr Levin, headmaster of City of London School.
Instead, the HMC will encourage its members to get involved with schools that are already being run by established academy chains Ark and the United Learning Trust (ULT).
"I did due diligence. I hoped they would be able to sponsor, but the obstacles are too great," Mr Levin said.
Template agreements that private schools could use to strike deals with Ark and the ULT, outlining the ways in which schools could be involved with an academy, will be sent to HMC heads in the next fortnight. The most ambitious private schools could make financial donations of up to #163;250,000, winning them the chairmanship of the governing body and a say in the name of the new schools, it will say. They would also be involved in the recruitment of the head and other staff. Other options include teacher exchanges, inter-school competitions or lending facilities.
"The academy chains will take on the pension deficits, benefit from economies of scale and take on the administration and financial side," said Mr Levin. "It will leave the independent schools free to help with teaching and learning.
"This idea should get rid of some of the scepticism surrounding becoming involved in academies. The chains would want a significant commitment, but this should be a more attractive option."
Mr Levin said he hoped the publication of the template agreement would spark interest from around 15 to 20 schools, although the geographical spread of the academy chains' schools was limited. Ark currently has just six primary academies and the ULT has three all-through academies and one stand-alone primary academy.
The decision to step back from school "takeovers" comes amid independents' general reluctance to fully sponsor academies.
Only six schools - including Wellington College and Dulwich College - have become lead sponsors of academies, despite years of political pressure. A further 27 have become co-sponsors or educational partners, with Eton College and Manchester Grammar School sponsoring free schools. Mr Levin has also spoken of his wish to sponsor an academy as an individual.
A spokeswoman for Ark said they were open to involvement from independent schools that were "centres of excellence" if it was practical for both sides.
Meanwhile, the president of the Girls' Schools Association, Louise Robinson, said that full academy sponsorship had never really been an option for her members.
She said that many informal relationships with state schools had already been going on for years. "Parents are paying for what we are delivering and we have to be accountable to them," she added.
A spokeswoman for the DfE said: "There is no reason why a pension deficit should be a barrier to schools sponsoring an academy. All pension funds have deficit recovery plans in place and an existing pension deficit isn't repayable immediately."
The drive for independent schools to sponsor academies aims to build on successful partnerships between private and state schools that have taken place over many years.
The independent Colfe's School in Lee, southeast London, has worked closely with Conisborough College in Lewisham for the past three years, taking six positions on its governing body.
Exam results have improved from 19 per cent A* to C grades at GCSE including English and maths in 2007 to 49 per cent in 2011.