If you're one of the five million people who was informed they had overpaid or underpaid tax in the last financial year, you may well find another letter is on its way with a further revised calculation from HM Revenue and Customs (HMRC).

Mistakes by HMRC left about 5.5 million people underpaying or overpaying their tax through the Pay As You Earn System (PAYE) in 2013-14.

About two million people were thought to have paid too little and were due to repay the money from April 2015, while a further 3.5 million who paid too much tax received an automatic rebate (see our Free Tax Code Calculator to find out if you're owed a tax rebate).

But now HMRC says it will be sending out "updated calculations" as previous ones were "incomplete". It won't tell us how many people are affected, but says letters will be sent out in the next six to eight weeks to those who are.

It does say, however, that the fault lies largely with employers. An HMRC spokesperson says: "The majority of the errors have happened because employers failed to make a final payment statement for the 2013/14 tax year, meaning our records were incomplete despite reminders that these submissions had to be made. We are sorry this has happened and we will issue corrected calculations in the next few weeks."

The mistake means anyone who paid too little tax could end up having to pay more or having some of what they owe wiped. Meanwhile those who overpaid could be owed more by the taxman, or could face having to repay some of what they've been given back.

HMRC adds that some people may end up owing nothing at all.

The issue came to light after The Daily Telegraph reportedly saw a leaked email concerning the issue, in which a select group of senior HMRC staff and accountants were told "thousands" of mistakes were made.

If you believe you may be affected, HMRC says people should check their P60 against the calculations received earlier this year, in the first instance. If you’ve lost your P60, you can request a copy from your employer or HMRC, or you could also check your final payslip from the end of the tax year to see if the amounts tally.

It's each individual's responsibility to check they are on the right tax code, you can check if yours is right using our Free Tax Code Calculator.