Keir Starmer has been accused of trying to price families out of private schools after he vowed to retain Jeremy Corbyn's policy of scrapping their charitable status.
The Labour leader has said he would use the tax revenue to fund a schools catch-up programme.
Tory MPs have called it a "spite measure" that would make private schools only affordable to wealthy foreigners and pile further pressure on Britain's state sector.
Last September, Sir Keir announced that he intended to pursue Corbyn's policy - raising £1.7 billion a year in tax from private schools which would be used to fund education in the state sector.
He pledged to end the charitable status enjoyed by many fee-paying schools, meaning they would lose their 20 per cent VAT exemption – worth £1.6 billion – and would have to pay £104 million in business rates.
A 20 per cent tax on fees would mean that tuition at top institutions could rise by almost £10,000 in some instances, with a £44,000 tuition increasing to nearly £53,000 a year.
A parent paying this yearly tuition fee for their child would have to find a total of £370,000 for a seven-year stint at secondary level.
Therefore, it is estimated that more than 90,000 pupils would have to switch to state schools because their parents would be priced out of school fees.
'It will become rich foreigners only'
Sir Keir was warned last night that the policy would cost more than it would raise.
Tory MP Sir John Redwood, who was in charge of Margaret Thatcher's policy unit, told the Daily Mail: "They are clearly trying to price people out of sending their children to private schools from the UK – it will become rich foreigners only."
He added: "It will therefore greatly increase the burden of public spending because all those people will need full-price state places free, instead of paying for the private-sector places themselves.
"It's a typical, vengeful tax to stop people getting on in the world which ends up costing all of us a lot more money and making many more people miserable."
Former education secretary Nadhim Zahawi has previously warned that this policy would flood the state sector with new pupils and would ultimately cost the taxpayer money.
He said: "I saw some of the analysis done on this – that actually if you do do that you will not increase tax take for the exchequer.
"Actually it will go the other way because many parents will come out of the independent sector and would add to the number of children that we will need to accommodate in the state sector."
The Chancellor Jeremy Hunt also took a swipe at the policy during his autumn statement earlier this month, branding it "ideological" and claiming it could cause 90,000 pupils to switch to state schools.
A Labour spokesperson said: "Labour is determined that all children, irrespective of their family income, have the chance to achieve their potential through high quality education.
"That's why the next Labour government will remove charitable status from private schools to fund our national excellence programme for all schools.
"We'll recruit thousands of new teachers, turn around struggling schools, and give staff the training they need to enable all children to pursue their dreams and reach their potential."
A Government spokesperson said: "Independent schools have an important role to play in levelling up opportunities."