The chief executive of HM Revenue and Customs has apologised to MPs for the poor performance of tax call centres between April and June, when only half of calls for help from the public were answered successfully.
Lin Homer said she was "very apologetic" after Mark Garnier MP, on the Treasury Select Committee, said the record was "staggeringly bad".
She added that after HMRC struggled to cope during those months, managers introduced a series of changes which resulted in an improvement between July and September when 76 per cent of enquiries by phone were dealt with either by a member of staff or an appropriate recorded message.
She said that performance during the second three month period was "marginally higher than last year".
Mr Garnier responded that it was still "completely unacceptable" that 24 per cent of calls were not being answered.
Lin Homer told MPs that a new telephony system had been introduced which allowed calls to be dropped into any call centre, so that up to 20,000 staff could be deployed to answer the phone at any one time.
And she revealed that more people were being taken on to deal with calls in the evening.