Martin Lewis has warned millions of workers could be owed thousands of pounds as they may be on the wrong tax code.
One viewer contacted the Money Saving Expert (MSE) founder to explain how she claimed £5,000 back after spending four years on the wrong code.
Lesley emailed in to say: "I read Martin's advice to check your tax code.
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"I'd never bothered to check as I was paid the same each month, but when I looked into it, for the past four years I was on the wrong code.
"Nice £5,000 refund on its way."
Tax codes are used by employers and pension providers to work out how much Income Tax to take from your pay or pension.
HM Revenue and Customs (HMRC) will tell them which code to use.
But millions are wrong each year, according to Mr Lewis, and the rules say it is your responsibility to check it is correct - rather than that of your employer or HMRC.
MSE has an easy-to-use calculator to check if you're on the right code.
A disclaimer says the calculator will only "give you a steer" and you'll need to back the results up with your own research, and contact with the tax office.
If your tax code is wrong, the first thing to do is to tell HMRC you think your tax code may be wrong, and why.
The easiest way of doing this is to call them on 0300 200 3300.
If you're on the wrong tax code, you will have likely been paying too much - or not enough - tax.
People who have overpaid tax are entitled to a full refund.
If it's the current tax year and you're paying too much tax as a result of your tax code right now, HMRC will inform your employer, the tax code will be amended and the overdue tax will be refunded to you via your wages.
People who have overpaid for a number of years have reported being reimbursed with a cheque.
What if I have underpaid?
In most cases the money will have to be paid back.
Generally HMRC can only go back four tax years, but this extends to six years if you acted carelessly and 20 years if you acted deliberately.
It might seem unfair that because you were put on the wrong code, you're liable for these repayments, but the official line from HMRC is that it is each individual's responsibility to check they are on the right tax code.
However, if your code was wrong there are some situations where you might not have to pay the tax bill.
This could include:
- If your employer made a mistake and put you on the wrong tax code despite being sent the correct one by HMRC. In that case, HMRC should try first to recover the tax from your employer.
- If HMRC reviews your tax and finds an underpayment of £50 or less in the last year, it'll write off the tax.
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