Tax chiefs have apologised to customers for poor performance and huge delays following a damning report by MPs.
Mike Clasper, chairman of HM Revenue & Customs, said sorry to taxpayers after the Commons Treasury committee uncovered a raft of problems with the service.
They included phone calls going unanswered at peak times and "endemic" delays in answering letters.
Mr Clasper admitted: "We are not happy with our performance in 2010."
He told BBC Radio 5 live: "It simply wasn't good enough on post and telephone and I'd like to take the opportunity to apologise to the people who had to take a long time to get through, or we didn't get back to them quick enough with the post.
"In 2011 we've been working very very hard to improve things. We're handling the calls immediately much more frequently than we did in 2010 and as far as individual customers are concerned you know the post levels have dropped in half.
"That's not where we want to be but it's a lot better than where we were in 2010."
It followed the release of the Administration And Effectiveness Of HM Revenue And Customs report, which concluded there was a "serious risk" that if communicating with the HMRC became too time-consuming, difficult and expensive, "respect for the tax system, and with it voluntary compliance, may be undermined".
MPs said the evidence they had received during their investigation was "disturbing" and found problems with bad management and demoralised staff.
The report said: "HMRC's delivery of services to