UK tax inspectors are investigating Goal.com, which bills itself as the "world's largest football website", over its widespread use of unpaid interns, the Guardian can reveal.

The site has confirmed it uses a roster of 30 unpaid interns across seven days a week to file match reports and write other content for its UK edition.

Its parent company, Perform, which is listed on the London Stock Exchange, says Goal.com attracts an average 24 million unique users a month and last year generated £4m in advertising and sponsorship revenue. Perform, which bought Goal.com for £18m in February last year, posted total revenues of £103m for 2011.

A copy of a Goal.com staff schedule seen by the Guardian appears to show between five and seven interns a day being used between 7am and midnight to file news and match reports. Perform told the Guardian it had 22 full-time staff, plus paid freelancers.

Following a push last year by the deputy prime minister, Nick Clegg, HM Revenue & Customs has been stepping up its investigations into unpaid internships, especially in the fashion industry, believing they may breach national minimum wage laws. Under the National Minimum Wage Act 1998 those who work must be paid, although charities and public authorities can be exempted to take account of volunteering.

This year interns have successfully petitioned for back pay from companies including the Top Shop owner Arcadia Group and most recently Harrods.

One former graduate, who wanted to be known as Keiron, said he worked as an intern at Goal.com for more than a year. "I guess it is because sports journalism is something that is so big, so popular, so many people like me want to get into it, that any 'in' is fine."

He started in September 2009 and worked a day a week for a year until he was finally offered paid work as a subeditor at the start of 2011. He said unpaid interns from Glasgow to Kent worked six-hour shifts from 7am and would email in their stories and communicate with paid editors via Skype. One company document seen by the Guardian has just over 60 contact details under the identification code "ukintern".

"I never actually met the editor who was in charge of [Goal.com] other than on Skype. I almost knew all along that something wasn't quite right," Keiron said. "My dad was always telling me, 'why are you doing this when you're not getting paid, you should be getting paid for it'."

Goal.com describes itself as the "world's largest football website" and says it has 400 editorial staff based in more than 50 countries across the globe "who produce an average of 1,500 news stories per day in 15 languages across 22 editions".

Perform declined to give a statement in response to the HMRC investigation, but told the Guardian it was proud of its internship programme. It said it had around 30 volunteer interns who looked after the UK edition, and it tried to give them as much or as little experience as they wanted.

It added that half of its interns had been to Premier League matches or press conferences as part of their work, and a majority of the full-time staff had started as interns.

It said it was hard to say whether it was reliant on interns to produce copy, and it felt it should provide the opportunity for experience in sports journalism because it had work available. It said it always paid incurred expenses.

Ben Lyons, a campaigner with Intern Aware, said the website's practices were exploitative. "We hear reports of appalling unpaid internship systems, but few companies are as bad as Goal.com in having more interns than regular employees."

He said that, at a time of extremely high youth unemployment, "it is wrong for Goal.com to take advantage of young people's desperation to get experience … without the guarantee of a job. As unpaid internships are often illegal, HMRC must launch a thorough investigation into Goal.com and the tens of thousands of unpaid internships springing up across the country."

An HMRC spokesperson said they could not comment on individual cases, but added: "HMRC ensure that businesses comply with the national minimum wage rules across the board, and where we believe these rules are being abused, we will not hesitate in investigating further. Where we receive information of possible [national minimum wage] abuse, we will always analyse it to identify anyone breaking the law."