A record £15.6m worth of underpayment to UK workers has been discovered by the government in the past year.
Social care, commercial warehousing and the gig economy were the sectors most likely to underpay staff, HMRC said.
It had prioritised checking these areas alongside apprentices, migrant workers and employment agencies, it said.
In total, the firms found to have underpaid workers were fined £14m as well as having to reimburse their staff.
The Low Pay Commission chairman Bryan Sanderson said: "All workers are entitled to be paid at least the minimum wage, so it is good to see increased focus on enforcement bearing fruit and securing more arrears for more workers."
The government said funding for enforcement was at record levels - rising to £26.3m in 2018/19 from £20m in 2016/17.
Minimum wage rates
21-24 year old rate: £7.38
18-20: £5.90
16-17 £4.20
Apprentice rate: £3.70
Business minister Kelly Tolhurst urged firms to check that they were getting their workers' pay right.
We are dedicated to stopping underpayment of the minimum wage. Employers must recognise their responsibilities and pay their workers the money they are entitled to," she said.
She credited the rise in the number of underpaid workers discovered to "a record government clampdown".
BBC News.
