Martin Lewis has issued a tax code warning to workers across the UK.

The Money Saving Expert founder predicts millions could be on the wrong tax code and due a refund of thousands of pounds.

One reader contacted the consumer champion and explained how she managed to claim £5,000 after being on the wrong tax code for four years.

What are tax codes?

Tax codes are used by your employer to work out how much tax is taken from your pay or pension.

Your tax code is a combination of numbers and letters representing how much you can earn before you start being taxed.

Everyone on the PAYE system should be issued one.

The most common code for the current tax year is 1257L for people who have one job or pension.

If you're on the wrong tax code, you could be paying too much or not enough tax each month.

In the most recent MoneySavingExpert email, Martin warned: "Millions are wrong each year, and the rules say it's YOUR RESPONSIBILITY to check it's correct (not your employer's or HMRC's)."

How to check your tax code

Looking at your payslip or P45 is the quickest and simplest way to check your tax code.

You could also ask your HR department if they can tell you your tax code.

The GOV.UK website has a webpage dedicated to allowing people to check their tax code. To check it online you will need to register for a government gateway ID.

For anyone on the most common 1257L tax code, that means you can earn £12,570 before being taxed in the 2021/22 tax year.

Checking if you're on the right tax code

MoneySavingExpert has a free tax code calculator to get a rough idea of whether your current tax code is correct.

To check yours, you will need to enter your earnings before tax and your current code, and the calculator will tell you if you're likely on the right code as no calculator can tell you for definite.

If you have recently started a new job or you have not given your employer enough information regarding your circumstances, you could be on the wrong tax code.

How to claim tax back if you have been overpaying

Depending on how much and how long you've been overpaying for, you could potentially claim thousands of pounds back.

By calling 0300 200 3300 to contact HMRC, you can investigate the best course of action for you, or you could speak to them online via their live chat service.

If your tax code is wrong for the current year, your employer will be contacted by HMRC and you should get back any owed tax in your next payslip.

You can claim back up to four years to the 2017/18 tax year.

If you think you have been overpaying much longer than this, you could get in touch with HMRC to investigate.

What happens if you've been underpaying tax

If you realise you've been underpaying tax, you will owe HMRC money.

To avoid paying back a larger sum, it's best to contact them straight away and let them know.

You could organise a payment schedule depending on your circumstances.

As it was not your fault, you could try and get the tax written off, although this is not a guarantee.

This could be done by asking for a so-called "Extra Statutory Concession" or an A19 from HMRC.