More than £15m has been paid to workers not receiving the full minimum wage in the past year, new figures show.
HM Revenue and Customs said a record 200,000 of the UK's lowest paid employees received money they were owed, twice as many as in the previous year.
The increase followed the launch last year of an online service by HMRC, which led to a 132% rise in the number of complaints.
Business Minister Andrew Griffiths said: "Employers abusing the system and paying under the legal minimum are breaking the law.
"Short-changing workers is a red line for this Government and employers who cross the line will be identified by HMRC and forced to pay back every penny and could be hit with fines of up to 200% of wages owed."
Penny Ciniewicz, the director general of customer compliance at HMRC, said: "HMRC is committed to getting money back into the pockets of underpaid workers."