Private school fees in minister's funding claims
The figures quoted by education ministers defending their record on state school spending also included the money spent by parents on private school fees.
This has been confirmed by the OECD think tank that compiles the international comparisons of spending figures.
Head teachers' leaders say the Department for Education is "disrespecting" schools and teachers by this "extraordinary" use of statistics.
The Department for Education accepts that the spending claim is not limited to public spending on schools - but it stands by its use of the figure as "accurate".
But the heads warn the department has "serious questions to answer" over its "veracity".
This follows the discovery earlier this week that the spending claims quoted by the Department for Education were also counting the tuition fees being paid by university students.
Education ministers last week faced accusations that they were failing to provide adequate funding for schools in England - with head teachers staging a protest in Westminster.
Ministers dismissed the claims made by school leaders - saying that not only were schools getting more money than ever, the UK was the "third highest spender on education in the world".
But this claim has faced intense scrutiny - with heads saying it was "shocking and disturbing" that the Department for Education could quote a spending figure that turned out to include billions spent by students on tuition fees.
On BBC Radio 4's Today programme last Friday, the School Standards Minister, Nick Gibb, argued against a head teacher warning of funding shortages.
"We are spending record amounts on our school funding. We are the third highest spender on education in the OECD," said Mr Gibb.
But what has angered heads is the discovery that this "third highest spender" is not really about government funding of schools, it includes all types of spending, for school and university, including tuition fees.
'Accurate' or 'distorted'
It has also now emerged that the Department for Education's spending claim includes the fees paid by parents in the independent school sector.
Jules White, organiser of the head teachers' funding protest last week, accused ministers of using "partial and distorted information".
Paul Whiteman, leader of the National Association of Head Teachers, said: "We have consistently argued that funding for schools is down by 8% in real terms. The National Audit Office agrees, the Institute of Fiscal Studies agrees, parents can see it is true, as can the electorate."
He said that head teachers would be "disappointed but not surprised that the department has serious questions to answer over the veracity of their claims".
"If trust goes, there is little left for the profession to hold on to. Failing to face up to the truth will cheat an entire generation."
Geoff Barton, leader of the ASCL head teachers' union, said: "We have also looked at the OECD data quoted by the Department for Education and it does seem to include spending on private education as well as on university tuition fees.
"It is extraordinary that the department used that statistic to defend its record on school funding. I think that school leaders and teachers feel that the DfE is disrespecting them when it uses statistics which don't bear any analysis."