The trade union Unite has written to HM Revenue & Customs asking it to investigate whether some of the UK’s largest charities are breaking the law by not paying the national minimum wage to interns.
The trade union, which has more than 60,000 members in the not-for-profit sector, said it had evidence that more than a third of the 50 largest employers in the charity sector used unpaid interns, based on a survey of job adverts placed by those charities.
Unite said it could not prove that charities were acting illegally by doing so, because of "ambiguities in the minimum wage law", but it urged HMRC, which has responsibility for enforcing the national minimum wage, to investigate.
It said it would not name the charities involved in the practice.
In May, Unite teamed up with the Intern Aware pressure group to publish a report, Interns in the Voluntary Sector – Time to End Exploitation. The report said charities should pay interns the minimum wage, otherwise "working for voluntary organisations will become the preserve of a wealthy elite".
Sally Kosky, national officer for the not-for-profit sector at Unite, said: "Many of these charities are multimillion-pound enterprises that can well afford to pay. Writing to HMRC is the next step in our campaign to stop these exploitative charities trading on the goodwill and compassionate nature of young people.
"HMRC is charged with enforcing the national minimum wage, and we would urge it to take action where it is clear that young people doing unpaid intern roles are actually doing proper jobs of work and should be paid accordingly."
An HMRC spokeswoman said the department investigated every complaint of possible minimum wage abuse.
"Paying the national minimum wage is not a choice – it’s the law," she said. "When employers – including charities – ignore those rules, we take steps to ensure arrears are paid out in full, and the employers fined."
HMRC recovered a total of £4m for 26,000 UK employees last year, said the spokeswoman, adding that criminal prosecution was "a last resort".