Nine in ten private school teachers at Britain's largest academy chain say they have had to take second jobs to make ends meet because they are paid so little.
They say United Learning's latest pay offer is so derisory in comparison to what their state school counterparts are paid that they were forced out on a landmark strike this week in protest.
Meanwhile unions representing them say they are being 'treated with contempt'.
Staff at Surbiton High School and Banstead Preparatory School downed tools after United Learning, which runs 90 academies and 14 independent schools, offered them pay rises of just two or three percent.
They are incensed that the group, which has net assets of more than £50 million, is offering them around half the statutory 5.5 per cent rise given to the academy teachers.
Now they are lobbying parents at both schools to write to governors and chief executive Sir Jon Coles, who was paid around £300,000 last year, to find out where their fees are going.
At present Surbiton High School, in south west London, has fees of up to £24,000 a year while Banstead Preparatory School, in Surrey, charges parents up to £19,000 a year.
But, according to teachers and unions, this fee income is not ring-fenced for individual schools but is spent across the group. One teacher, who was on the picket line at Surbiton, told the Mail on Sunday: 'The reasoning doesn't make sense.'
At present Surbiton High School, in south west London, has fees of up to £24,000 a year while Banstead Preparatory School, in Surrey, charges parents up to £19,000 a year.
One teacher, who was on the picket line at Surbiton, told the Mail on Sunday: 'The reasoning doesn't make sense.'
Staff at Surbiton High School and Banstead Preparatory School downed tools after United Learning offered them pay rises of just two or three percent.
'The pay is so bad that 90 per cent of teachers are having to take second jobs, such as tutoring, to make ends meet. And, the younger staff, who cannot afford to live near the school, are not getting enough salary to pay their travel.
'It is really tough. Every single person on strike doesn’t want to be on strike because they love teaching. They enjoy being with the children; they enjoy teaching; they enjoy education.
'There is also no transparency. Parents pay the fees, but it goes into a pot of money for all their private schools, and as teachers we don’t feel that it’s going back into the school.
'We’re trying to get parents to hold United Learning accountable. They are really supportive on the whole.'
National Education Union (NEU) senior organiser Rachel Evans said: 'Once again, United Learning have offered their independent schoolteachers a pay award significantly below both inflation and the increase afforded to teachers in the state sector.'
'By our estimation, this award will mean that the majority of United Learning independent schools offer pay lower than that in local state schools.'
This week's strike came after talks broke down with ACAS and NEU members after ten of United Learning's 14 independent schools voted in March in favour of industrial action.
The move comes in the wake of the news that one of the group's top schools, the £40,000 a year Royal School, in Haslemere, Surrey, is due to close in August. Financial pressures such as Labour's VAT on fees policy are believed to be behind the closure.
NASUWT Acting General Secretary Matt Wrack added: 'United Learning Trust is treating our members with contempt and paying them less than teachers in the state-funded academies they run.'
'How can it claim teachers at Surbiton High School are worth less than teachers in their other schools? ULT should not treat hard-working and committed teachers at Surbiton in this appalling way.'
United Learning was approached for comment.