The customer service of HM Revenue & Customs (HMRC) is now so unacceptably poor it could be considered a "genuine threat to tax collection", according to the accounts auditor.
The tax office answered just 50% of calls during the first half of 2015, compared to 73% in 2014/15, and 74% in 2011/12, which was still described at the time by the Public Accounts Committee (PAC) as an "abysmal record on customer service".
HMRC also answered fewer than two in five (39% of) calls within five minutes last year.
The committee says HMRC's standard of customer service is "unacceptable", and adds that it's particularly concerning because people are more likely to pay the right tax when they find HMRC easy to deal with.
It recommends HMRC sets a more challenging short-term target for call-waiting times, and a long-term target that is much closer to industry standards, as its target of answering 80% of calls within five minutes is "woefully inadequate and unambitious".
The PAC adds that HMRC should identify what impact its poor level of customer service is having on tax revenues, and produce a detailed plan setting out how and when it will provide an acceptable level of customer service.
Meg Hillier MP, PAC chair, says: "It beggars belief that, having made disappointing progress on tax evasion and avoidance, the taxman also seems incapable of running a satisfactory service for people trying to pay their fair share."
The news comes after HMRC announced in June this year it was setting aside £45 million to improve customer service, after figures showed 7.2 million phone calls made last year went through to a 'busy' tone.
HMRC says it plans to modernise the services it provides customers through better use of technology; for example, by introducing an online renewals system for tax credits claimants.