Further wages rises are planned

The national minimum wage was introduced four years ago and now stands at £4.50 an hour, or £3.80 for trainees and workers aged between 18 and 21.

A report by the TUC and the Low Pay Network estimates that 170,000 workers are still being paid less than they are entitled to by law.

Other employers are cutting hours or upping demands on productivity, leaving workers with no more cash, say low pay campaigners.

Repaid wages

The TUC is publishing a government-backed booklet giving advice on the minimum wages workers should receive.

Since the minimum wage was introduced in 1999, lawbreaking employers have been forced to pay up £13m in lost wages.

There must be no hiding place for rogue bosses Brendan Barber TUC general secretary

TUC general secretary Brendan Barber said those figures showed the Inland Revenue's enforcement teams were doing a good job.

"But TUC figures suggest that there are many more workers out there still losing out," he said.

"There must be no hiding place for rogue bosses. Our guide aims to make it easier for unions and advice agencies to root them out and ensure everyone gets the wage to which they are legally entitled."

The report says the worst-hit workers are most likely to be in the clothing, footwear and retail industries, or work as hairdressers, domestic workers or housekeepers.

Workers from overseas employed in farming or food production are also affected, it says.

Time cuts

Kate Gordon, training officer for the West Midlands Low Pay Unit, told BBC Radio 4's Today programme some employers paid the legal hourly rate.

But "they then go on to cut someone's hours or they increase their productivity targets dramatically so that in the end the worker does not actaully earn any more money", she said.

Employment Minister Gerry Sutcliffe said: "Clearly it is unacceptable not to pay the national minimum wage.

"People know its in place, we are going to continue to get the message across of what people's rights are."

Bosses failing to pay the proper rate would all have to pay back to their employees and former employees money from 1999, he said.

"It will cost them a lot more," added Mr Sutcliffe.