HMRC met its target to handle 80% of calls in only 10 weeks of the year, the NRO report said.

Its performance deteriorated further over the first seven months of 2015-16. Average waiting times tripled compared to 2014-15 levels, peaking at 47 minutes for self-assessment callers during the deadline week for paper returns in October 2015.

The NAO estimates the overall cost incurred by customers who called the taxes helpline increased from £63m in 2012-13 to £97m in 2015-16. The estimate includes call charges at £10m, the value of customers’ time spent waiting to speak to an adviser at £66m and value of time spent talking to advisers at £21m.

Within this estimate, customers paid £2m less in call costs because HMRC reduced call charges by moving from higher-rate numbers to local-rate 03 telephone numbers in September 2013. An increase in the economic cost of time spent waiting for an answer or speaking to an adviser more than offset the saving, the NAO said.

Ruth Owen, HMRC’s director general for customer services, said the authority has recognised that it did not provide an adequate standard of service but has since improved.

“Over the past six months we’ve consistently answered calls in an average of six minutes, and have launched new online tax accounts and web chat for everyone,” she said.

HMRC Customer Service telephone number.