Sarah Butler cites Sutton Trust research on internships in the Guardian.
The government has launched a crackdown on unpaid internships, sending more than 550 warning letters to companies and setting up enforcement teams to tackle repeat offenders.
HM Revenue & Customs is expected to target sectors such as media, the performing arts and law and accountancy firms, which have a reputation for using unpaid interns.
It will also issue guidance to employers spelling out when they are legally obliged to pay at least the national minimum wage to interns.
About 70,000 internships are offered each year in the UK, according to the Sutton Trust social mobility charity. It estimates that of 10,000 graduates who are in internships six months after they leave university, a fifth are unpaid. It has calculated the cost of doing an unpaid internship as more than £1,000 a month in London and £827 in Manchester, putting valuable work experience beyond the reach of those from families on low and middle incomes.