Workers and pensioners are being warned to double-check their tax codes or face being landed with shock bills that they cannot appeal.
More than 20 million codes, which tell you how much of your income will be tax-free, are sent to 16 million people at this time of year.
But already The Daily Mail has seen several cases where the code - a combination of letters and numbers - is wrong.
Mistakes typically occur when someone has several sources of income or where they have received a large one-off boost, such as a dividend payment, in the previous year.
This can leave someone paying too much or too little tax in future years. The problem is compounded because a rule that allows taxpayers to fight unfair bills is being quietly thrown out by HM Revenue & Customs, according to the Daily Mail.
Tax bosses want to overhaul a special concession used by hundreds of thousands of people each year to contest debts they believe have been caused by mistakes at HMRC.
The new rules make taxpayers themselves directly responsible for checking their code is correct and spotting mistakes. If they fail to do this, their appeal will not be upheld.
This will make it all but impossible to claim, since the vast majority rely on their employer or pension provider to take care of tax matters.
Previously, to qualify for an appeal the mistake had to have been spotted within 12 months of the end of the tax year, and the people owing the cash had to have had no reasonable way of knowing they were underpaying.
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