A report by the National Audit Office (NAO) into the customer service performance of HM Revenue and Customs has highlighted that 6.5m customers had to wait more than 10 minutes have their call answered in the first half of this year, while 20m calls were left not answered at all.

The NAO estimates that it cost customers £33m in call charges while they are in the queue. The estimated value of customers’ time while they are in the queue is £103m.

Amyas Morse, head of the NAO, said: “The taxpayers and claimants who phone HMRC do not have a choice about whether they interact with the department.

“Despite some welcome improvements, HMRC has acknowledged that its performance in providing services to the public has been unacceptable.

“HMRC faces difficult decisions about whether it should aspire to meet the service performance standards of a commercial organization. It could do only by spending significantly more money or becoming substantially more cost effective.”

The NAO says that so far in 2012-13, HMRC has improved its handling of post but its performance in handling calls has been varied. However, in October 2012, HMRC answered 91% of calls, its highest monthly performance since December 2009.

Most of HMRC’s numbers are still 0845 numbers which result in high call charges for some customers.

It is investigating alternatives as it negotiates its new telephony contract. HMRC estimates customers would save £13m annually if all 0845 numbers were replaced with cheaper 03 numbers.

HMRC has plans to achieve service targets but they are said to be ambitious, given previous performance and the need to reduce staff.

NAO analysis indicates that, by the end of 2012-13 and through 2013-14, HMRC could achieve its target of answering 90% of calls.

However, by 2014-15, HMRC will have reduced numbers of contact centre staff so will need to redeploy large numbers of back-office processing staff to answer telephones.

There is also uncertainty about the impact on call volumes of large-scale changes, such as the introduction of Real Time Information and the transition to universal credit.