Meanwhile, other workers may be losing out as they have unpaid working time, like having to get to work early to open the premises.
What has the MoneySavingExpert said?
MoneySavingExpert has updated its guide on the minimum wage, including ways to check if you are getting paid less than you should.
For example, if you are required to buy anything to carry out your job, like tools or a uniform, your workplace has to ensure this does not mean your wage falls below the legal requirement. This applies however the cash is deducted.
It includes buying items in a shop, paying your boss for them or having cash taken from your salary.
If you get minimum wage, any time you spend working you should be paid for - including overtime, time spent in training, travelling time and time spent opening up or closing down a workplace.
On Twitter, Lewis said: "Are you on minimum wage? You may be underpaid, 500,000 are! Many who think they are on minimum wage are in reality actually being paid less than they should."
How to check if you are underpaid
If you think you might not be getting the full minimum wage, the government has a national minimum wage and living wage calculator to help you work out what you should be getting. You can also talk to your boss informally to try to get things resolved.
Another option is to put in a formal complaint to your employer if that does not work, or you can take the issue to the Advisory, Conciliation and Arbitration Service (Acas), a government-backed body that steps in with workplace issues.
You can contact Acas by filling in an online form or by calling 0300 123 1122.
Alternatively, you can contact HM Revenue & Customs (HMRC) by filling in form online.
You will have to tick ‘yes’ when asked if you are willing to be contacted by the taxman, as otherwise they may not be able to investigate.
If HMRC discovers you were not paid the proper minimum wage, it can order your employer to repay the difference, for the past six years. It can also fine the employer and take them to court if they do not pay up.
How has the minimum wage changed?
Employees who get minimum wage should get a pay increase from April every year under law.
From April this year, the hourly rate for earners aged 21 to 2 is £9.18 an hour, up from £8.36. The National Living Wage for over-23s is up to £9.50.
The apprentice rate has also risen, up from £4.30 an hour to £4.81.