Supply teacher agencies are denying teachers their “basic legal rights” by using offshore companies to avoid paying tax and national insurance, a union warned today.

The NASUWT has surveyed more than 1,400 supply teachers and found that two-thirds had been asked by an agency to sign a “contract or agreement with an umbrella or offshore organisation”.

Chris Keates (pictured), the union’s general secretary, said: “Supply teachers are paid with taxpayers’ money and it is scandalous that this is being siphoned off by offshore umbrella companies who are seeking to avoid paying tax and national insurance by exploiting supply teachers and schools.”

New anti-tax avoidance measures are due to come into effect to tackle the issue this month. The government says it has noted a growth in recent years “in the use of offshore employers to employ UK workers, working in the UK, for UK based companies” and has used the teacher supply industry as an example of the problem.

The NASUWT said supply teachers are also underpaid, with 56 per cent of respondents to the poll saying they were “not paid at a level commensurate with their experience level”.

“My wage has been cut between 20-25 per cent in the last 10 years (depending on the agency I work for),” one supply teacher said. “I find I am contacted by other agencies supplying the same schools offering more work if I take a wage cut.”

The union will debate a motion today that denounces some unnamed companies and agencies for using zero-hour contracts “which impact detrimentally on the lives of supply teachers and their families”.

Its survey found that nearly 7 per cent of supply teachers did not always have access to toilets.

The NASUWT has introduced a new website - supplyadvisor.co.uk - which allows teachers to rate agencies and write reviews on them.

“The NASUWT is deeply concerned about the exploitation supply teachers are routinely facing at the hands of unscrupulous agencies,” Ms Keates said.

“This website facility puts supply teachers back in control. It will be a powerful tool to enable supply teachers to influence the market, shining a spotlight not only on the poor practices of bad agencies, but also highlighting those agencies that treat their teachers well.

“Supply teachers are often simply too frightened to speak out about their treatment by some of these unscrupulous supply agencies due to threats of ‘blacklisting’.

“This is compounded by the fact that many supply teachers are being asked to sign contracts that include gagging clauses designed to cover up a range of illegal practices. No working person should be treated in this way.”