The most common one is 1257L and the way to work this out is to add a zero to the end which becomes £12,570 - their tax-free allowance.

He warned “It is crucial to check your tax code as millions of them may be incorrect, and it is neither HMRC nor your employer is responsible for verifying the correctness of your tax code; it is entirely your responsibility.

“If you have the wrong tax code, it could mean you might be entitled to get cash back if you have overpaid, but might also mean you have to pay some money back if you underpaid."

How to check your tax code

People should have recently received a letter from HMRC with their new tax code.

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