Martin Lewis ’ MoneySavingExpert website has urged workers to check their tax code is correct - or risk overpaying by thousands of pounds.
Your tax code is used by HMRC to determine how much tax you should be paying each month.
It is made up of numbers and letters to represent how much you can earn before you start being taxed.
But MSE claims millions of people are on the wrong tax code without realising - if this is you, you could be owed money back from the taxman.
This week (April 6) marked the start of the new tax year, so it’s worth checking now if your tax code is correct.
MSE said: " Your tax code is a series of letters and numbers (for example, 1257L) which tells your employer or pension provider what tax to deduct.
"However, millions of codes could be wrong."
The biggest change you’ll notice on your payslip is an increase of how much National Insurance you’re paying.
National Insurance contributions have just increased by 1.25 percentage points, from 12% to 13%.
The threshold for when you start paying has also just gone up from £9,568 to £9,880 - but this is due to rise again, and by a bigger jump, from July.
The rate at which you start paying will go up to £12,570 from July 6 - meaning more low income workers will keep more pay in their pockets, although not for a few months yet.
How to check your tax code
You should have recently received a letter from HMRC ahead of the new tax year with your new code.
Otherwise, you can find your tax code on your latest payslip, or on your P45 if you have recently quit your job.
Gov.uk also has a dedicated webpage when you can see your tax code.
Have you managed to claim back £1,000s from HMRC? Let us know: mirror.money.saving@mirror.co.uk
To check it online, you will need to register for a government gateway ID - this is free to do.
The most common code for the current tax year is 1257L for people who have one job or pension.
But not everyone should be on this tax code - for example, people who have more than one job.
MoneySavingExpert has just updated its free tax code calculator so you can get a rough idea as to whether yours is correct.
To check yours, you need to enter your earnings before tax and your current code.
No tax calculator will be able to tell you for definite if you are on the right code - but this should give you a good indication.
How to claim back tax
If you suspect you've been overpaying, you can contact HMRC on 0300 200 3300. You can also speak to an advisor online via their live chat service.
HMRC will contact your employer to correct your tax code, and you'll get back any owed tax in your next payslip.
If you have been overpaying tax for some time, you can claim back up to four additional years - so as far back as the 2018/19 tax year.
But if you have been underpaying, you will need to pay your tax back.
It is better to sort this out sooner rather than later to avoid having to pay back a larger sum.
How much you pay back, and how often, will vary depending on your circumstances.
You can also try and get the tax written off if it was not your fault that you underpaid - but this is not a guarantee.
You can do this by asking for a so-called "Extra Statutory Concession" or an A19 from HMRC.
