Martin Lewis is urging the millions of people in the UK who receive a regular payslip to check it immediately.
The Money Saving Expert founder was live in Liverpool on Tuesday for an end of series special of his Money Show Live on ITV.
The finance guru said people should check their wage slips as soon as possible before an April deadline as they could lose out on thousands if pounds if they don't.
READ MORE: Martin Lewis warning to anyone with a mobile phone contract
He told his audience, and ITV viewers, that if the deadline is missed and it turns out your tax code was wrong, you won't be able to reclaim the money and some people could lose thousands of pounds that they're owed. Yorkshire Live reports he also told people exactly how to check their tax codes.
Mr Lewis said: "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'."
He added: "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.
Receive newsletters with the latest news, sport and what's on updates from the Liverpool ECHO by signing up here
READ NEXT:
Additional help for people struggling to pay energy bills
Emmerdale fans say character has had 'personality transplant'
Miller and Carter's strict dress code sees man turned away
Doctor 'bowed her head' as she read baby's test results to parents
