Tax inspectors will be writing to dozens of top flight British football clubs to warn them they must pay the minimum wage to a retinue of match staff including mascots, physio interns and ball boy supervisors, or face a £5,000 fine and potential prosecution.
HM Revenue and Customs will send letters to 44 Premier League and Championship clubs on Tuesday warning that its inspectors will begin a series of "targeted checks" after receiving evidence about workplace exploitation in some of the UK's wealthiest businesses.
HMRC said it was taking "pre-emptive action" at the start of the new season to put a stop to unlawful activity and said its intelligence included complaints about non-payment from more than one current serving club mascot.
Revenue and Customs said it could not give further details but in April Reading and Swansea were found to be advertising for unpaid interns, including one post which lasted a year and did not even include reimbursement for expenses.
Under national minimum wage laws it is illegal not to pay those classed as workers and there is no separate exemption for those undertaking work experience.
Scores of adverts for unpaid positions have appeared on the website of UK Sport, a quango that has received public and lottery funding to the tune of £500m.
A performance analyst at Swansea claimed he was not paid despite working days of more than 12 hours on occasion and having to meeting tight deadlines for the club. Swansea FC said it was now paying all of its staff, including its mascot, Cyril the Swan.
On the eve of HMRC's actions, Reading FC said the club stood behind a previous statement that unpaid internships were an "important part of career progression and experience building for any individual starting out on the path to their dream job".
However it added that it did pay its mascot and clarified: "It goes without saying we will always comply with HMRC regulations."
In HMRC's letter, the head of dynamic response team for the national minimum wage (NMW), Gabriel Murphy, said: "We will shortly start to carry out a series of targeted checks of employers within the sporting industry.
"We strongly recommend that you check whether you are treating any interns, volunteers or work experience workers correctly for NMW purposes. And that you are paying your workers at or above the current rate of NMW.
"If you find that you are not treating any interns or work experience workers correctly, or that you have not been paying the correct rate of NMW, you should put matters right straightaway.
"If we select you for a check and find that you have not correctly dealt with these matters, and you have not put things right, we may charge you a penalty of up to £5,000."
Lower down the league, Portsmouth FC, which recently went into administration for a second time, revealed last month that it has been using unpaid jobseekers from the government's mandatory work experience scheme to act as cleaners and maintenance workers in the stadium.
The club, which is now owned by its fans, told its local paper last month that it had 35 people working six and a half hours a day, amounting to a total of 3,000 hours of "painting and maintenance and general cleaning". If the jobseekers refused they would have their benefits stripped.
Speaking to Portsmouth News, Derek Stone said: "They get something out of this, but we get something too … we're painting things that haven't seen a lick of paint in maybe 25 years."
Ben Lyons, from campaign group Intern Aware, said: "Football should be about fair play off the pitch as well as on it. It is astonishing that top football clubs have been taking advantage of young people who want a career in their industry by refusing to pay them a penny.
"While writing to clubs is a good first step, HMRC must use its powers to secure unpaid wages for ex-interns, and it should prosecute the worst offenders. The business department, which is responsible for interns, must also begin naming and shaming companies that illegally refuse to pay interns."
Michelle Wyer, assistant director of the NMW team, said: "Paying the national minimum wage is not a choice: it's the law. It can't be right that as some players are paid millions of pounds, other members of staff are paid below the legal limit.
"HMRC enforces the rules, protecting workers from rogue employers and ensuring they get at least the wage to which they are legally entitled. Where an employer ignores these rules, we will take steps to ensure arrears are paid out in full and the employer fined. In the most serious cases, criminal prosecution can follow."
• This article was amended on 20 August 2013. An earlier version said that there is now separate exemption [under national minimum wage laws] for those undertaking work experience. In fact, there is no separate exemption for work experience. "Now" has been changed to "no" as it materially affects the accuracy of that sentence.