Thousands of workers are being short-changed by firms who refuse to pay the national minimum wage, a survey for union organisation the TUC found.
The minimum wage is £5.52 an hour for adults
Around 150,000 staff are being denied rate of £5.52 an hour for adults and £4.60 for 18 to 21-year-olds, it says.
Those in restaurants, hotels, cleaning, hairdressing and childcare were said to be the most likely to be underpaid.
A TUC helpline for staff has been set up. Earlier this month, ministers said firms would face tougher penalties.
Companies will now risk unlimited fines, with the most serious cases of non-compliance tried in a Crown Court. The way arrears are calculated will also change, boosting the amount of back pay due to underpaid employees.
The TUC said the exploitation was "unacceptable", especially at Christmas when workers face greater financial pressures.
TUC general secretary Brendan Barber said: "There should be no hiding place for bosses who are deliberately cheating their workers out of the minimum wage.
"If you know somebody who is not being paid the minimum wage, now is the time to help them. If you know a Scrooge employer who is not paying workers a decent wage for a fair day's work, now is the time to report them."
The TUC said it had been contacted by a homeworker paid between £2 and £3 an hour, and a Polish man working in a pizza store for a similar wage.
Employers can be reported to the minimum wage helpline on 0845 6000 678.