Six per cent of calls to HM Revenue and Customs' (HMRC) telephone helpline service resulted in taxpayers receiving less than complete and correct advice, while the percentage of calls answered within 20 seconds fell seven per cent short of the industry benchmark standard at 73 per cent.
Furthermore some of the guidance material provided required a reading age of 16 or 17, despite the fact that less than half the adult population has this level of literacy.
"Tax can be a complicated matter so it is important to make it as easy as possible for taxpayers to understand and comply with their tax obligations," NAO chief Sir John Bourn said.
"My report shows how forms and guidance can be made more accessible and the importance of grasping opportunities to simplify the complex rules and procedures."
The report calls for a redesign of the HMRC website, a drive to improve the assessment process for face-to-face advice and renewed efforts to improve the call-answering rate.
It cited recent HMRC research showing that unintentional errors from taxpayers completing their forms resulted in lost revenue totalling over £300 million.
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