The gap between private-school fees and state-school per-pupil spending in England has more than doubled over the past decade, the Institute for Fiscal Studies says.
Average fees, of £13,600, were more than 90% higher than the £7,100 spent on state-school pupils in 2020-21, compared with a gap of 39% in 2009-10.
For sixth-formers, fees are about three times higher than per-student funding.
The government says schools are having the biggest funding uplift in a decade.
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While private-school fees have grown by more than 20% above inflation since 2009-10, state-school per-pupil spending has fallen by 9% in real terms.
And the gap in resource levels is probably even larger, the IFS researchers say, as the figures do not include other forms of income for private schools, such as account investment and endowments or gifts.
"Longstanding concerns about inequalities between private- and state-school pupils, which have come into sharp focus during the pandemic, will not begin to be easily addressed while the sectors enjoy such different levels of resourcing," Luke Sibieta said.
Labour's shadow education secretary, Kate Green, said: "School budgets have been hammered over the last decade, which is holding children back.
"As state-school class sizes have soared and enriching activities - art, sport, music, drama - have been cut back, the gap with private schools has grown ever wider."
Association of School and College Leaders general secretary Geoff Barton said: "It is pretty outrageous that the government has cut funding in real terms to schools and colleges over the past decade, while independent school fees have increased over the same period.
"The funding gap between the two sectors has always been there of course but the fact it has widened to such a huge extent does stick in the throat.
"Surely the government should want the same opportunities for all children and young people.
"It may be naive to think that state education funding could match the independent sector but it surely shouldn't actually go into reverse."
A Department for Education spokesman said: "This government is providing the biggest uplift to school funding in a decade - £14bn in total over the three years to 2022-23.
"This includes a £7.1bn increase in funding for schools by 2022-23, compared to 2019-20 funding levels.
"Next year, funding through the schools national funding formula (NFF) is increasing by 2.8% per pupil compared to 2021-22.
"The NFF continues to distribute this fairly, based on the needs of schools and their pupil cohorts."