Almost one in three internships on a government website for graduate job-hunters this year has been unpaid, a Freedom of Information request shows.
In a political row earlier this year Deputy Prime Minister Nick Clegg called for interns to be paid.
But the Graduate Talent Pool website, set up by the Department for Business, Innovation and Skills (BIS), continues to offer many unpaid internships.
A BIS spokesman said businesses should be open and fair about what is offered.
University leavers face tough competition in the jobs market - and internships are seen as a way of getting a foot on the ladder.
But a Freedom of Information request, submitted by a graduate recruitment company called Give A Grad A Go, shows that 30% of the internships advertised so far this year on the government's official Graduate Talent Pool website have been unpaid.
Up to 6 May, more than 2,200 vacancies advertised on the website, out of a total of more than 7,600, were unpaid.
The website itself shows that out of the 2,239 vacancies currently available, only 1,235 are paid vacancies.
Among the unpaid internships are full-time posts lasting for several months, with a number of agencies appearing to use the website to recruit such unpaid interns.
The Graduate Talent Pool website was set up by the government as a way of connecting graduates with internships being offered by businesses.
But there have been criticisms over how internships can be misused as unpaid labour by some employers - and warnings about the need for them to comply with the minimum-wage regulations.
There have been particular warnings about the impact of the intern system on social mobility - with claims that it gives an unfair advantage to the well-connected and those who can afford to work without being paid or only receiving minimal expenses.
Mr Clegg highlighted his concerns in his social-mobility strategy - sparking a political row about his own access to work experience and differences of opinion with the prime minister.
"We want to improve understanding of the application of national minimum-wage legislation to internships and ensure that employers comply with it," said Mr Clegg's social-mobility proposals.
"Where an individual is entitled to the minimum wage they should receive it and we take failure to do so very seriously."
The Graduate Talent Pool website also warns employers that: "If an intern is effectively performing as a 'worker', then in most cases they will be entitled to the national minimum wage."
Cary Curtis, managing director of Give A Grad A Go, said: "Recent social-mobility debate between the prime minister and his deputy has highlighted the lack of a coherent approach to internships and work placements.
"Nick Clegg's new Social Mobility Strategy is a step in the right direction, as we believe all graduates should be treated as employees and paid accordingly, regardless of the position's label or their social standing.
"The word 'internship' carries no legal definition and therefore often leads to graduate exploitation," said Mr Curtis.
He also criticised the finding that the government department did not keep a record of how many graduates found a job through its internship website.
A spokesperson for the Department for Business, Innovation and Skills said: "We encourage businesses to offer internships openly and transparently and to provide financial support to ensure fair access.
"Some interns don't qualify for the national minimum wage and we encourage employers to pay reasonable out-of-pocket expenses in these cases."
A spokesman for the Deputy Prime Minister urged employers of interns to pay heed to minimum wage legislation, and to consider paying the minimum wage or out of pocket expenses to "ensure fair access".
"But this is as much about opportunity as it is about money," the spokesman said.
"Too often, such opportunities can only be taken by well off, well connected families," he said, adding that the deputy prime minister "welcomes schemes like the Graduate Talent Pool that provide an open and transparent means for people from any background to find the right opportunity for them".