Workers are being urged to make sure they are paid the new national living wage after research revealed a third of people never check their payslips.
The new rate of £7.20 an hour comes into force on April 1 for workers aged over 25, giving more than a million low-paid workers a wage rise.
Some will have a pay increase of £900 because of the new rate, which compares with the national minimum wage of £6.70 an hour.
There have been reports of some firms cutting back on overtime or weekend payments in advance of the new rate coming into force.
Research by the government found that a third of the lowest paid workers do not check that they are being paid the correct rate, especially people in the North West and East of England.
Stewart Gee of the conciliation service Acas said: "Eligible workers should check what they are entitled to under these new changes and employers need to ensure that they are ready too."
Many firms are already paying a voluntary living wage of £9.40 an hour in London and £8.25 in the rest of the country, with French energy giant EDF the latest to be accredited.
A manager at retail giant B&Q has claimed the firm is removing time-and-a-half pay for working Sundays, stopping paying a bonus, and restructuring allowances for working in high cost of living areas, under moves to cut costs ahead of Friday.
"B&Q are asking people to sign their new terms and conditions of employment or they will be dismissed," he wrote on the change.org website.
A B&Q spokesman said: "Our aim is to reward all of our people fairly so that employees who are doing the same job receive the same pay.
"That isn't the case at the moment as some have been benefiting from allowances for a long time when others have not and that can't continue.
"Over 12 months ago, long before the national living wage announcement, we commenced a review of our pay and reward framework introduced in 2004, and these changes reflect that review.
"B&Q is committed to being a good payer and our overall package is one of the best in retail, with benefits that include a performance bonus, market-leading pension scheme with up to 14% employer contribution, a ShareSave scheme and five weeks holiday a year.
"Overall, we will be paying more this year than we did last year.
"From April our minimum rate will be £7.66 per hour."
Frances O'Grady said: "The TUC fully supports a higher minimum wage.
"However, we don't think it's fair that under-25s will be on a lower hourly rate.
"They face many of the same expenses as other adults and should be paid the same.
"If the government wants to attract more young people into apprenticeships, then it needs to up the £3.40 apprenticeship rate.
"Future minimum wage increases must do more to narrow the pay gap between old and young."
Kamaljeet Jandu, national officer of the GMB union, said: "Those employers trying to get around the minimum wage by changing conditions of employment are shooting themselves in the foot."
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