Martin Lewis has urged anybody who gets a payslip to check it right away, as they could be at risk of losing thousands of pounds if they don't.
Appearing on his Money Show Live on ITV on Tuesday, March 21, the finance guru was discussing tax codes as there's an important deadline coming up in April.
If the tax code on your wage slip is incorrect and you don't notify HMRC in time, you might lose out on a lot of money that you're owed. To help people out, Martin explained exactly how to check your tax code and what the different letters and numbers mean.
Yorkshire Live reports that he told people how to check their tax code, advising viewers: "Urgently check if yours is correct. Your tax code is a number that tells your employer or your pension provider what tax to deduct from your income.
"1257L is a common one. The way to work this out is add a zero to the end, so that becomes £12,570 which is your tax-free allowance.
"As for the L, that's complicated. I think from memory, that means 'lower'.
"My uncle, Tax Tony, who is a just retiring tax accountant, is that right?
He said: "L is for unmarried people as in a lower increase in an allowance each year. H is for married people to take into account a wife's income as additional allowance."
Martin then added: "You can see it runs in the family can't you!
"So, you need to check yours. Millions are wrong and it is YOUR responsibility. It is not HMRC's responsibility, it is not your employer's responsibility.
"If it's wrong and you underpaid, they'll come and they'll want the money and that is a nightmare for people.
"But if it's wrong and you overpaid then you're due the money back, but the deadline for the 2018-19 year is coming fast. This could be big money."
The deadline for the 2018-19 year is April so you don't have long to claim the money if you're owed.
Martin also advised that anyone who wears a uniform for work - even just a branded t-shirt - could be eligible to claim a rebate of £60 a year on their taxes, but you only have days left to check before the 2018-19 year deadline passes.
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