Millions of people could have paid the wrong tax last year after becoming fed up with waiting to talk to HM Revenue and Customs advisers.
Callers to HMRC were forced to wait for an average of 47 minutes when paper tax returns were due last October - causing one in five to hang up.
In a bid to fix the problem, HMRC transferred 2,400 staff to man the phones at call centres who were performing essential work to maintain PAYE records.
But this caused the number of discrepancies between PAYE and self-assessment tax returns to double - leaving a risk that 3.2million people have paid the wrong amount of tax.
Public Accounts Committee chair Meg Hiller MP told the Daily Telegraph: “The fact that they are hanging on the phone waiting is not good enough.
“Most taxpayers just want to pay the tax they owe on time, without questions. If they can’t get access to the right advice they are the ones who pay the penalty not the HMRC.”
HMRC’s general director for customer services Ruth Owen said that the service has fully recovered since early 2015.
"We recognise that early in 2015 we didn't provide the standard of service that people are entitled to expect and we apologised at the time," she told the Telegraph, adding: "We have since fully recovered and are now offering our best service levels in years."
Whitehall auditors have accused HMRC of underestimating the demand for telephone contact when they moved 5,000 employees away from call centres.
An NAO survey of taxpayers found one in five rated HMRC’s performance as poor or terrible.
The Evening Standard has contacted HMRC for comment.
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