The number of workplaces being inspected to see if they are paying staff the minimum wage has more than halved under the coalition Government, prompting warnings that exploited workers are not being protected.
There were just 1,455 employer inspections last year, a 60% fall from the 3,643 carried out in the final year of the last Labour government, Treasury figures showed.
Staff were paid less than the minimum wage in nearly half (47%) of the workplaces inspected in 2013/14, up from 34% of all employers investigated in 2009/10.
The proportion of inspected workplaces found to be paying staff less than the minimum wage has steadily risen since the coalition came to power in 2010, while the number of inspections has fallen.
There have only been two prosecutions in the six-year period, but HM Revenue and Customs (HMRC) usually enforces full payment of wages through the civil courts if the employer does not pay arrears.
Shadow attorney general Emily Thornberry, who uncovered the figures using a written parliamentary question, criticised the Government for not doing enough to protect workers from "rogue employers".
She said: "These alarming figures show that the Government is not doing enough to protect people from being exploited and undercut.
"The number of workplace inspections has collapsed since Labour was in office and 2013-14 was yet another year to have passed without a single prosecution.
"HMRC haven't even referred a single case to prosecutors for the last three years.
"A Labour government would overhaul this Government's derelict enforcement regime by doubling fines for rogue employers and treating worker exploitation with the seriousness it deserves."
In his reply to Ms Thornberry, Exchequer Secretary David Gauke said: " The Government take the enforcement of the national minimum wage (NMW) very seriously and HMRC reviews every complaint that is referred to it, investigating the complaint and, in addition, carrying out targeted enforcement where we identify a high risk of non-payment of NMW.
"The majority of employers identified as paying below the NMW will pay arrears on receipt of a formal notice of underpayment. Where they don't, HMRC will pursue recovery through the civil courts.
"For deliberate non-compliance or obstructive behaviour, HMRC operates a policy of selective and exemplary criminal investigation action to bolster our overall enforcement strategy.
"Criminal investigations rarely result in arrears being paid to workers and are reserved only for the most serious cases."
The standard national minimum wage for over-21s is £6.31, decreasing to £5.03 for 18-20 year olds, and £3.72 for under-18s.
:: The number of inspections and the number of workplaces found to be paying staff less than the minimum wage between 2009 and 2014 is outlined below:
2009/10: 3,643 completed inspections, 1,256 non-compliant workplaces.
2010/11: 2,904 completed inspections, 1,140 non-compliant workplaces.
2011/12: 2,534 completed inspections, 968 non-compliant workplaces.
2012/13: 1,693 completed inspections, 736 non-compliant workplaces.
2013/14: 1,455 completed inspections, 680 non-compliant workplaces.