Phil Kemp
The BBC's 5 live Investigates programme has discovered that thousands of supply teachers working in the UK are paid by Sark-based International Subcontracting Solutions Ltd (ISS).
ISS is a payroll company - sometimes known as an "umbrella" company - which pays the salaries and expenses of workers who find jobs through recruitment agencies in the UK.
The arrangement means that temporary workers, such as supply teachers, are the employees of ISS.
Because ISS is based offshore it does not pay employer's National Insurance contributions - but neither do the UK-based recruitment agencies that find the jobs for staff paid by ISS.
An offshore company employing thousands of teachers is avoiding the payment of millions of pounds in employer's National Insurance contributions.
ISS Ltd, based in the Channel Islands, employs more than 24,000 temporary agency workers across the UK, most of them working as supply teachers.
ISS says it is "meticulous in complying with HMRC codes on taxes and expenses".
HM Revenue & Customs says schools, councils or employment agencies could be liable for the shortfall.
This could add up to many millions of pounds in unpaid tax. For example, for a supply teacher on a daily rate of £160, around £90 per week is not being paid to HMRC in employer's National Insurance contributions.
