An analysis by the Higher Education Careers Services Unit (HECSU) found that less than 1 per cent of graduates were taking the controversial placements.
Critics have claimed that unpaid internships exclude young people who cannot rely on their families to support them financially during the placement, and so limit social mobility.
A total of 1,835 graduates who left university in 2011-12 were known to be in unpaid internships six months later, HECSU said, and they were clustered in the design, publishing and media, advertising, and public relations industries.
Six out of ten were in London, but the practice was hardly reported at all in Scotland, Wales, Northern Ireland and the North East of England.
Master’s graduates (0.5 per cent) were slightly less likely to be taking unpaid internships than first degree graduates (0.6 per cent).
The conclusions draw on data from the 2011-12 Destination of Leavers of Higher Education survey.
Charlie Ball, HECSU’s deputy director of research, said: “For the first time this data gives us hard evidence of the nature and extent of unpaid internship for new graduates.
He added: “Ideally we would like to see this figure as low as possible with everybody who is classed as a worker paid fairly.
“What’s clear is that there remain many industries that still have a job to do in terms of cleaning up their employment practices.”