More than 26,000 workers paid less than the national minimum wage have received £4m between them after investigations by HM Revenue & Customs, figures show.
HMRC said 708 employers had been fined with charges of up to £5,000, after it reviewed 1,693 complaints in 2012-13.
Affected workers were given an average of £300 in back-pay, it said.
Employment Minister Jo Swinson said paying people less than was legally owed to them was "totally unacceptable".
The national minimum wage is currently £6.19 an hour for workers aged 21.
Cases investigated by HMRC included the illegal use of interns, unpaid extra hours and workers forced to buy company clothes as uniform.
In one instance, a major fashion chain was ordered to pay its 90 unpaid interns almost £60,000, HMRC said.
A retailer which "required employees to purchase specific items of clothing from its range" was ordered to repay almost £170,000 to more than 6,000 workers.
Another company, described as a "national retailer", was forced to pay more than £193,000 to 3,500 staff for requiring them to attend work before and after opening hours without pay.
Liberal Democrat minister Ms Swinson said: "Whenever we find examples of businesses breaking the law we will crack down on them."
"Supporting fairness in the workplace is one of our key priorities and the national minimum wage is one way of making sure this happens."
She added: "It supports as many workers as possible without damaging their employment prospects, which is why effectively enforcing the minimum wage is critically important in making sure it stays a success."
Trades Union Congress general secretary Frances O'Grady said: "These investigations show why more resources must be put into catching minimum wage cheats.
"As well as handing out fines, the government must publicly name and shame all those rogue employers who knowingly underpay their staff."
Meanwhile, information published by the Department for Business, Innovation and Skills indicated there was "no definition of an internship/work experience in minimum wage legislation".
But it said the arrangement a person had with an employer might mean they were a "worker", therefore entitling them to the minimum wage, even if they were described as an "intern".
The government's checklist of what "generally" describes a worker includes the promise of future work and having to turn up for work if you do not want to.