The Government could cut schools spending without compromising standards, an independent think-tank has claimed.
The link between a bigger budget and better student performance is a myth, according to the authors of a Reform report called "Must do better".
The body said 18% could be slashed from the schools budget at the next Parliament without this affecting quality of teaching.
Kimberley Trewhitt, co-author of the report, said: "This analysis breaks new ground by looking at primary schools as well as secondary.
"Lifting the ring-fence on the budget will help schools think harder about how best to use their resources. Their top priority should be improvement in the quality of their teachers as this is what really matters for pupil performance."
The report compared funding of almost all primary and secondary schools in the country to pupils' achievement in English and Maths and found there was no link between them.
It also failed to find a correlation between a bigger budget and better quality of teaching.
While they admitted the idea of cutting the ring-fenced budget would be controversial, the authors claimed the results of their survey showed many schools are over-funded.
They said spending on both primary and secondary education increased by 86% in real terms between 2002 and 2012.
They also suggested less emphasis should be placed on class sizes and the number of teaching assistants, which in England has increased from 79,000 in 2000 to 232,200 in 2012.