Independent schools cannot deliver effective help to state schools with "a gun pointing at our heads", one of the country's leading headteachers will say today.
Mike Buchanan, chairman of the Headmasters' and Headmistresses' Conference (HMC), will say that colleagues are largely in support of Theresa May's plans to improve state education, but that "forcing" the public and private institutions to work together is "fraught with difficulties".
Speaking at the annual conference in Stratford-upon-Avon, Mr Buchanan will compare the Prime Minister's vision for changing the way schools work together with an arranged marriage.
He will say: "Independent and state schools cannot make our relationships work with a gun pointing at our heads. We hope the Prime Minister understands that - after all, she had the good sense to outlaw forced marriages as Home Secretary.
"She must know, then, that all good partnerships are based on mutual desire, understanding, respect and cooperation. They work best when the parties have a good deal in common.
"Sustainable partnerships also require some down-to-earth, practical things to be in place - such as proximity. The ability to drop by, share experiences and talk through problems is a great asset as the most successful multi-academy trusts know.
"Thus, in the messy, complex real world, forcing independent and state schools together is fraught with practical difficulties and, ultimately, is unlikely to work."
Last month, Mrs May outlined plans to increase the number of "good" school places.
She said: "What we want to do as a Government is ensure that universities are taking more of a role in supporting schools and opening schools, that there's more of a contribution from the independent sector, that we increase the number of faith schools and, yes, that we lift the ban on people setting up or expanding selective schools."
The chairman of the HMC, which represents 282 of the UK's top independent schools, will pledge to support the Government's aims to improve state sector education and offer more places in independent schools to those who cannot afford full fees.
However, he will stress that huge amounts of good work goes on behind the scenes already.
Mr Buchanan, headteacher at Ashford School in Kent, will tell conference delegates: "We know that our colleagues in state schools often do a fantastic job with fewer resources, larger classes, more curriculum constraints and significantly different challenges and we do not presume to patronise them by suggesting we can necessarily run their schools better than they can.
"But with open-hearted collaboration and a flexible approach, great things can happen, and I am hopeful the PM's evidence-based and practical approach will prevail.
"The further expansion of subsidised school places is bound to be the right choice for a good many HMC schools and coercion is unnecessary."
A Department for Education spokeswoman said: "We recognise that many independent schools already have successful partnerships with state schools and we welcome the pledge from the independent school sector to continue to help to drive further improvements in the state education system."
A three-month consultation has opened on the DfE website over the plans to create more good school places.
The spokeswoman said: "We believe independent schools could and should do more in recognition of the tax benefits they receive.
"That is why we are proposing that independent schools with the capacity and capability should sponsor academies or set up a new free school, or offer a proportion of places to less wealthy children.
"We are urging everyone to look at the detail in the consultation, join the debate and have their say."