More than 1.5 million workers are being "cheated" out of the national minimum wage by dishonest employers, with hairdressers, hotel and bar staff most likely to suffer, according to new research.
The TUC said workers across the country are still taking home less than the legal minimum wage because of rogue employers who were paying below the adult rate of £5.73 an hour.
Workers being cheated out of the minimum wage are most likely to be in London, the North West and Merseyside, or the South East, and employed in hotels, bars and restaurants, retail or hairdressing, said the report.
TUC general secretary Brendan Barber said: "The Revenue and Customs teams responsible for enforcement are doing a good job and have already retrieved millions of pounds from employers failing to pay the minimum wage.
"But TUC research suggests that there are at least 1.5 million workers across the UK who are still losing out. This shows the need for continued public funding for the national minimum wage helpline which allows people to make a complaint about an employer. There must be no hiding place for rogue bosses."
The report revealed details of under-payment, including a 32-year-old migrant paid £4.10 an hour for construction work, a North East homeworker on just £1 an hour and a former UN peacekeeper in Bosnia who moved to the UK to learn English who was paid £2.40 an hour at a poultry processing factory.
The minimum wage for over 21-year-olds is £5.73 per hour, for those aged 18 to 21 the rate is £4.77 per hour and for 16 and 17-year-olds it is £3.53 an hour.
The body which recommends the level of the national minimum wage has asked the Government if it can delay this year's suggested rise, it was announced.
Ministers have accepted a request from the Low Pay Commission to extend the recommendation to May, instead of February.
The commission said it wanted to study the Bank of England's inflation report, employee job figures for December, GDP figures for the fourth quarter of 2008, and updates on average earnings.
