HMRC looked into almost 1,700 complaints in the past year, leading to more than 700 employers receiving penalty charges of up to the maximum of £5,000.
More than 26,500 workers received an average of £300 in back pay, topping up wages that had been below the legal minimum of £6.19 an hour for adults.
Cases included a major fashion chain ordered to pay 90 unpaid interns almost £60,000; a retailer which forced staff to buy clothes from its range, ordered to repay almost £170,000 to more than 6,000 workers, and another retailer which required employees to attend work before and after opening hours without pay, told to pay more than £193,000 to 3,500 of its staff.
Employment Minister Jo Swinson said: "Paying less than the minimum wage is totally unacceptable. 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. 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.
TUC general secretary Frances O'Grady commented: "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.
"Nearly a million UK workers rely on the national minimum wage, which has become a vital lifeline. There must be no hiding places for companies who flout it.
"The action taken by HMRC is a welcome step but must be the beginning of a concerted campaign that also raises awareness about the right to a legal wage among those being exploited."