In many cases, those undertaking unpaid internships were able to do so because they received money from parents, lived at home or with family or friends, or used savings, the Sutton Trust's research found.
Chiera Mclaughlin, a fourth year law student at the University of Glasgow, said the decision for many was "balancing whether you want to get experience in the industry you want to work in, or if you have money for food".
"There's not that freedom to accept unpaid positions," she added.
From her experience in the law industry, she said that paid internships were "rare" and "to get any early work experience in law, it's expected that you wouldn't get paid for it".
The Sutton Trust's research also found that internships were almost twice as likely to have been found through family and friends in an organisation compared to an advertisement, further contributing to the widening access gap.
"If I was trying to get experience in say corporate law or at a law firm of some sort, it seems really difficult to even get your foot in the door if you don't maybe know someone there," Chiera agreed.
"I'm the first person from my family to go to uni and I don't really have any family connections, or anyone in my social circle, that's in the industry."
As part of its 'Make Work Pay' plan issued before the election, Labour said it wanted to ban unpaid internships, aside from those that form part of an education or training course. Details on when that might happen have not been confirmed.