The country’s largest academy chain, Academies Enterprise Trust (AET), is considering outsourcing most non-teaching roles to private companies in a deal worth up to £400 million.

The 10 year contract would include school business managers, librarians and IT staff as well as a range of back office positions such as HR, finance, and secretaries.

Headteachers, teachers and teaching assistants would be the only roles not affected by the move.

Unison, a union that represents 240,000 school support staff, has issued an angry warning about the move, claiming it is an “unprecedented privatisation of school services”.

The contract was advertised on the European Union’s online tender service and the deal will be worth between £200 and £400 million.

Jon Richards, Unison’s head of education, claimed the move would affect at least 500 staff across the chain’s 80 academies, adding the involvement of private companies would mean cutbacks.

“This move to effectively remove the control of most of the school from head teachers and to put profits before pupils is a dangerous decision, and we are urging AET to stop this privatisation process,” Mr Richards said.

“Privatisation on this scale could see the head teacher lose direct control of critical services, with the potential for significant leakage of public funds into private sector profits.”

In a statement to TES, AET said it had a “duty” to show value for money when it came to taxpayers’ money, adding the proposed changes would not impact the ability of heads to run their schools.

“As an exempt charity, accountable to both the Secretary of State and the Charity Commission, we have a commitment to maintaining the quality of school services, and we reinvest all surpluses into the running of our schools,” the sponsor said.

“The AET board has therefore agreed that we issue a tender to seek expressions of interest from organisations who may be able to work with us to provide more efficient, effective and economic services. Our aim is to seek a partner that will bring additional expertise, experience and capital to strengthen our operations.

“We will maintain the quality of the service by setting out very clear specifications, and remaining directly involved in the management of the service.”

And it added: “We have proactively engaged with the affected unions to share with them our proposals. We have been very clear about the scope of the procurement, as set out in the publically available procurement notice. The legal structure of the company to be majority owned by AET will be finalised in discussions with potential partners.”