Labour party members have voted to commit the party to integrate private schools into the state sector.
The motion calls for funds and properties held by private schools to be “redistributed democratically and fairly” to other schools.
It says universities would have to admit the same proportion of private school students as state school pupils.
But the body which represents private schools said the overhaul would be an “act of national self-harm”.
The vote means members have signalled a desire for the policy to be included in the next Labour Party general election manifesto.
Speaking at the party’s conference in Brighton, the shadow education secretary Angela Rayner said “tax loopholes” that benefit private schools would be scrapped by a Labour government in its first budget,
She told the Labour Party conference that the money saved would “improve the lives of all children”.
This includes the withdrawal of charitable status, other public subsidies and tax privileges.
Ms Rayner said she will task the Social Mobility Commission – which the party would rename the Social Justice Commission – with “integrating private schools”.
“We will set that commission to making the whole education system fairer through the integration of private schools,” she said.
“Myself and John McDonnell will set out further steps the Labour government will take, but I can say today that our very first budget will immediately close the tax loopholes used by elite private schools and use that money to improve the lives of all children.”
Proposing the motion at the party’s conference, Ryan Quick said the education system must offer fair opportunities for all and not reward a privileged few based on their parents’ wealth.
The “old boys’ network” originating in private schools was holding the country back, he said, and the media was failing to challenge the “false consensus” on the issue.
He called for the “wonderful resources” that private schools had at their disposal – including historic endowments originally intended to help the poor – to be made available to all.
Ex-teacher John Wiseman, a member of the Unite union, said the number of privately educated MPs in the cabinet showed the extent of the problem facing the country.
“How can it be right in 21st Century Britain to still have a feudal education system where a privileged few receive tax-subsidised education on the back of ordinary working people?
“But rather than abolish these aberrations, this government continues to push further privatisation through the academy and free schools network.”
The Independent Schools Council said parents would be “rightly worried” at what Labour was proposing, saying it “put politics before the interests of children”.
“The move is an attack on the rights and freedoms of parents to make choices over the education of their children,” said its chief executive Julie Robinson.
“This decision is an ideological distraction from dealing with the real problems in education.
“We all want to see more funding for state schools and greater support for underperforming pupils, which is precisely why we encourage all schools to work together in the interests of every child.”
On the second day of its conference, Labour also unveiled a plan to scrap Ofsted and replace it with a new school inspection system.