HMRC uses your tax code to determine how much pay you should get each month.
It is made up of numbers and letters which tell your employer or pension provider what tax to deduct.
But the team behind Money Saving Expert (MSE) have suggested that “millions of codes could be wrong”.
It comes as national insurance contributions have increased by 1.25 percentage points from 12 per cent to 13 per cent.
The threshold for when you start paying has risen from £9,568 to £9,880 and is due to increase to £12,570 in July,
How to check your tax code
You should have recently received a letter from HMRC with your new tax code.
But if you didn’t, you can find your tax code on your latest payslip or on your P45 if you recently quit your job.
You can also find it on gov.uk/tax-codes but first must register for a government gateway ID.
Use the MSE free tax code calculator
MSE has updated its tax code calculator which people can use to check if their tax code for this year is correct.
To check yours, you just need to enter your earnings before tax and your current tax code.
The most common code for the current tax year is 1257L for people who have one job or pension.
How to claim back tax
If you think you are on the wrong tax code, you can contact HMRC on 0300 200 330 or speak to an advisor online via their live chat service.
HMRC will contact your employer to correct your tax code and you will can get any money you overpayed in tax in your next payslip.
You can also claim back up to four additional years if you have been overpaying for some time.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies