Companies have paid former interns hundreds of pounds after they petitioned to be paid the minimum wage.

Celebrity magazine NOW, a south London auction house and a major international media conglomerate have paid out more than £2,250 after two interns demanded to be given a wage for over seven-and-a-half months of work.

The case follows several other settlements made without recourse to the courts, including those of interns working for clothing retail group Arcadia, which owns Top Shop and Miss Selfridge.

Both interns were offered payouts after the Justice for Interns campaign, run by jobs website Graduate Fog and Intern Aware, took up their cause.

A 27-year-old graduate of Central Saint Martins College of Art and Design who wanted to be known by her middle name, Anita, said she had been paid around £750 for a one-month internship last year at NOW.

Anita said that although she attended a few fashion shoots, she spent most of her time working in a stockroom "packing and unpacking" clothes with another intern.

"It wasn't what I signed up to do … Generally you weren't treated very well."

She added that she was shown the ropes by one intern and didn't meet management staff until a week into the placement. She then went on to train her intern replacement.

The magazine's publisher, IPC media, said the reason for the settlement was that Anita had progressed to paid tasks.

In a statement it said: "IPC has a very clear policy covering work experience, internships and short-term freelance contracts. The individual undertook what had been understood by both parties to be unpaid work experience, but, during that time, it appears that she progressed to carrying out tasks for which she was entitled to be paid.

"Once this was brought to our attention, the payment was made through IPC's payroll."

IPC also disputed the term internship and said that it was usual for work experience staff to hand over to each other, though they would always meet with staff at the start of their placement. "This was a work experience placement. This individual was not an intern. Work experience staff already progressed into their placement may be asked to explain to newer work experience staff normal process etc. Passing on knowledge already gained and best practice is also a valuable part of the work experience," it said.

Anita, who was on jobseeker's allowance during this period, went on to do a six-month unpaid internship at a south London auction house, where she was told to clean and vacuum the office and catalogue items that were being put up for sale.

"It was all the jobs you'd have to do at an auction house but people were getting paid to do. I was doing them, and being asked to do them," she said.

"They basically led me on to believe that when a place became available I would be offered the job … if I stayed on and worked hard and impressed them I would up for the position."

However, after five months of full-time unpaid work, the company failed to inform her that they were offering a paid job and subsequently gave the position to an outsider.

"I felt a bit stupid. Even though I was working there, I had to work [unpaid] alongside someone who'd just joined."

The company eventually offered her £1,000 in a settlement, a sixth of her minimum wage claim.

Last year government lawyers warned ministers that many employers were breaking national minimum wage rules by not paying interns, advising that "most interns are likely to be workers and therefore entitled to the NMW".

"People should think carefully before doing internships because you can be really misled," Anita said. "I've done a few now, so I know better, but I think everyone should claim their money back. It's only fair. You put in the hours but you end up with nothing."

A second intern, who could not reveal his name or that of the company he worked for without breaking the terms of a post-settlement gagging agreement, said that he had worked for two weeks for a major US-owned media conglomerate, during which time he was doing exactly the same work as other staff and was offered very little training.

In a settlement letter sent last month the company wrote "they were very surprised" to receive a letter "stating that you feel you are entitled to be paid".

"However," the letter said, "the company is willing to pay you the sum of £486.40, which amounts to the minimum wage for the hours which you worked."

News of the payouts also follows the publication of the third and final report from the government's social mobility tsar, Alan Milburn, on Wednesday. Milburn said he was concerned by the "number of universities whose careers services advertise unpaid internships", and recommended that universities should no longer "facilitate any exploitative work placements of any kind".

Tanya de Grunwald, founder of Graduate Fog and one of the organisers of the campaign said: "These payouts are hugely significant for our campaign against unpaid internships.

"Companies are realising they don't have the right to unlimited free labour just because someone is young and desperate for experience. After years of being downtrodden and voiceless, interns are finding the courage to stand up to the tight-fisted employers who thought they could get something for nothing without any repercussions.

"Interns contact us every day with new stories – some of whom have worked for very famous companies and individuals – so we look forward to more victories like this in the next few weeks. It's going to get ugly."

Gus Baker from Intern Aware added: "We are delighted to see NOW magazine and big companies coughing up and paying young people ... But it's a strange state of affairs when the government is failing to enforce its own minimum-wage law, and pressure groups are left to fill the gap."

• This article was amended on 19 October 2012 to correct grammatical errors.