Labour would add VAT to private school fees immediately if it wins power at the next general election.
The party said it does not plan to “phase in” the change over several academic years if it enters Downing Street.
Instead, it is understood it will end the tax breaks enjoyed by private schools as soon as possible, as first reported by the i newspaper.
Labour leader Sir Keir Starmer has vowed to remove the charitable status from independent schools, which gives them at least 80% relief on business rates.
Labour says the policy could raise £1.7 billion to invest in state schools.
A party spokesman said: “While this out-of-touch, out-of-ideas, tired Tory government tinkers at the edges, briefing about plans they may or may not see through in five years’ time, Labour makes no apology for relentless focus on how to drive high and rising standards in our state schools.
“Because we are the party of fair taxes, we will end the unjustifiable tax break afforded to private schools and fund recruitment of over 6,500 more teachers and put access to mental health counselling in every school.”
