HMRC has admitted millions more people than previously thought could be affected by recent PAYE problems.
It had previously been estimated around six million people were impacted by the problem, which could lead to demands for extra payments or refunds.
However, giving evidence before the public accounts committee yesterday, Dame Lesley Strathie, HMRC's chief executive, admitted up to ten million cases have not been looked at yet, meaning the actual number could end up significantly higher.
She said: "The numbers will keep moving. Each time we do another batch we test assumptions against reality and judge what we think is in the rest of the mix.
"The ten million are those cases that are not completely straightforward. They are different scenarios and we are looking at them one at a time."
HMRC's problems were revealed last month, when it emerged millions were not paying the right amount under the PAYE system.
Around 1.4 million could face demands to make further payments, although 4.3 million have paid too much tax.
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