Brits are being asked to check their payslips after the new tax year and National Insurance changes.
Martin Lewis, founder of MoneySavingExpert, warned workers to check they have not been overcharged by the tax office.
Your tax code is used by HMRC which determines how much contributions you're required to pay each month.
It is made up of numbers and letters to represent how much you can earn before you start being taxed.
If you've had a pay packet since April 6, you should check to see if you're on the right tax code.
And if it's wrong, you might be able to reclaim cash that you've overpaid to HMRC.
It could also determine whether you're underpaying tax to help "avoid a future nightmare", Martin warned.
Workers who are unsure what code they should be on could look at their payslip, P45 or P60 form.
Alternatively ask the HR department at work if you need help in locating it.
National Insurance contributions have just increased by 1.25% points, from 12% to 13%.
Your tax code could be wrong if you've recently changed jobs or have more than one income to recently moved to different hours.
Workers can always check their tax code on the GOV.UK website where you'll be required to enter your government gateway ID.
The most common code for the current tax year is 1257L for people who have one job or pension.
Over on Martin's MoneySavingExpert website you can use free tax code calculator to work it out.
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If you feel like you're overpaying, you can contact HMRC or speak to an advisor online via their live chat service.
HMRC will contact your employer to correct your tax code, and you'll get back any owed tax in your next payslip.
How much you pay back and how often will vary depending on your circumstances.
You can also try and get the tax written off if it was not your fault that you underpaid but this is not a guarantee.
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