Independent schools cannot deliver effective help to state schools in England with "a gun pointing at our heads", a leading headmaster has said.
Mike Buchanan told the Headmasters' and Headmistresses' Conference that "forcing" them to work together was "unlikely to work".
He compared the government's plan for changing the way schools work together to a forced marriage.
Ministers say many private schools have good state sector partnerships.
The Headmasters' and Headmistresses' Conference (HMC) represents 282 of the UK's top independent schools.
'Unlikely to work'
As part of its proposals for secondary schools in England, the government has said independent schools will be able to maintain their charitable status and tax breaks only if they set up or sponsor a local state school.
Smaller private schools will be expected to send teachers to take lessons in state schools and the independent sector must offer more scholarships and bursaries to pupils who cannot afford fees.
Speaking at the HMC's annual conference in Stratford-upon-Avon, Mr Buchanan, headmaster at Ashford School in Kent, said: "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 co-operation. They work best when the parties have a good deal in common."
He argued that "sustainable partnerships" require "practical things", such as proximity, to work.
"The ability to drop by, share experiences and talk through problems is a great asset as the most successful multi-academy trusts know," he said.
"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, Prime Minister Theresa 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."
Mr Buchanan pledged to support the government's aims to improve state sector education and offer more places in independent schools to those who could not afford full fees.
He said: "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."
A Department for Education (DfE) 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."