The UK statistics watchdog is investigating the government over claims it is spending record amounts on school funding, after it emerged its figures included billions of pounds of university and private school fees.

For years, the Department of Education has parried calls to increase the cash available for schools by claiming funding is at record levels and the third-highest of the OECD group of rich nations.

Minister for school standards, Nick Gibb, repeated the claim last week in a bid to deflect criticism as over 2,000 headteachers marched on Downing Street to protest against funding cuts.

“We are spending record amounts on our school funding. We are the third-highest spender on education in the OECD,” he told the BBC’s Today programme.

The OECD, a group of the world’s 36 top economies, has ranked the UK as spending the third-highest proportion of its economic output on primary to tertiary education, at 6.2 per cent.

But the UK Statistics Authority has launched an investigation after analysis by the BBC found the government’s figures, from 2015, actually included spending on all educational institutions, not just schools.

It also includes private spending, including billions in fees for universities and private schools.

“The UK Statistics Authority and the Office for Statistics Regulation are investigating the concerns raised, and will publish their findings shortly,” said a spokesperson.

Headteachers reacted angrily to news of the government’s spending claims, which come as many schools are struggling to balance the books.

Figures from the Education Policy Institute released earlier this year revealed that the number of secondary schools in England running at a loss had nearly trebled.

Jules White, coordinator of the Worth Less? campaign, which organised last week’s march, called the government’s claims “shocking and disturbing”.

“The Department for Education continually uses partial and distorted information to mislead parents. Now there is irrefutable proof that this has happened yet again,” he said.

The investigation also comes as the government is reviewing how it measures the costs of the university student finance system, including whether student loans should appear as a cost in the public finances.

In a statement, the DfE reiterated its position and made no mention of the investigation.

“It is true to say that the OECD has ranked the UK as the third-highest for education funding - this includes tertiary and private education for every country,” it said.