Labours plan to put VAT on private school fees amounts to levelling down, according to the headteacher of the famously progressive Bedales School.
Will Goldsmith said it was unfair to punish private schools for a lack of funding in the state sector, adding that cash raised by the tax is unlikely to make much difference.
He also warned the fall-out from the dangerous policy risked harming recruitment of overseas pupils, as families may perceive it as a sector in trouble.
Mr Goldsmiths intervention comes after he addressed the annual conference of the HMC, which represents the UKs most elite private schools, this week in Belfast.
His comments are significant since £46,000-a-year Bedales, in Hampshire, prides itself on being liberal and progressive.
Speaking to the Daily Mail, he admitted that while the school is seen as Leftie, he cannot support the tax, which Labour says will pay for 6,500 state school teachers.
Will Goldsmith (pictured) said it was unfair to punish private schools for a lack of funding in the state sector, adding that cash raised by the tax is unlikely to make much difference
Lily Allen performs onstage at the O2 arena in London on May 17
The £46,000-a-year Bedales, in Hampshire, prides itself on being liberal and progressive
He said: Ultimately, the state sector is grossly underfunded and its not going to get better overnight.
Recruiting a paltry number of teachers is not going to improve things.
They need a much longer-term plan, and this is just a quick political hit.
Bedales, known for being unconventional and creative, has educated a host of stars including singer Lily Allen, actor Daniel Day-Lewis and presenter Kirstie Allsopp.
The irony is not lost on me that we are seen as Leftie, Mr Goldsmith added.
But he stressed: It feels like the Government are saying, if only some people can have it, then we would rather no-one had it.
It is a levelling down not a levelling up.
Mr Goldsmith said they should be looking at a longer-term project not to price independent schools out of the market but to raise the level of state education to the point where it is no longer relevant for people to need to use independent education.
He said: Theyre getting the order wrong for what they want to do.
Mr Goldsmith said he had spoken to many state school heads who are against the new tax themselves.
He said: When I talk to heads in the state sector, they are not pushing for VAT in schools - they have no illusions about the fact that as a policy it is not going to deliver what they say it is going to deliver.
They dont have a beef with the sector.
Pictured: Daniel Day Lewis at The National Board of Review Awards Gala on January 11 in New York
Bedales, known for being unconventional and creative, has educated a host of stars including singer Lily Allen, actor Daniel Day-Lewis and presenter Kirstie Allsopp (pictured)
And Mr Goldsmith, who spoke at the international section of the HMC conference, also said the policy is threatening one of the few successful export markets we have as a country.
According to the latest census by the Independent Schools Council (ISC), which covers the majority of the sector, pupils from abroad make up 4.6 per cent of those at private schools - or 25,469.
In other countries, the British private school system is seen as the best in the world, but Mr Goldsmith said Labours attack has put this reputation at risk.
He said: There is a perception internationally that the sector is in trouble and there are absolutely risks with the eco system that exists.
Looking at the independent sector in the UK without seeing it in the international context is a danger.
The new tax is due to be imposed in January, with many schools already admitting they will pass on the cost to parents by raising fees.
Some schools, especially those that cater to less wealthy families, fear the tax will cause an exodus of pupils, hitting fee income.
The Government has been contacted for comment.
A Government spokesman said: We want to ensure all children have the best chance in life to succeed.
Ending tax breaks on private schools will help to raise the revenue needed to fund our education priorities for next year, such as recruiting 6,500 new teachers.