Delays in answering phone calls to government tax hotlines are costing callers £136m a year, an investigation by the government's public spending watchdog reveals.
Some 20 million calls to HM Revenue & Customs (HMRC) hotlines - many of which are 0845 numbers - were not picked up at all last year, according to the National Audit Office (NAO).
Callers who did get through were also waiting longer to speak to an adviser - an average of nearly five minutes compared to just over a minute and a half in 2009/10.
But performance is "substantially worse" during busy periods, with some 6.5 million people left holding on for longer than 10 minutes in the first quarter of this financial year.
The NAO found that there had been some progress since thousands more staff were drafted in, with the 74 per cent pick-up rate significantly higher than the 48 per cent recorded in 2010/11.
Expensive for callers
But the report warned that the figures probably underestimated the issue, as calls are counted as answered even if they do not reach an adviser.
The fact that many of these hotlines are 0845 numbers makes it more expensive for callers, because these are not normally included in mobile phone call plans.
My concern is that the cost of hanging on the line hits those who call from a pay as you go mobile the hardest. Margaret Hodge, public accounts committee
"Depending on the tariff they pay their phone company, customers are charged once their call is connected even if they are held in a queue," the report said.
"We estimate that in 2011/12, it cost customers £33m in call charges while they waited for HMRC to answer the phone and the estimated value of customer time while they waited was £103m.
"We estimate that if HMRC improved performance to answer 90 per cent of calls and reduced waiting times, it could save customers around £52m a year," the report said. "HMRC currently plans to spend £34m to achieve this level of performance."
Public accounts committee chairman Margaret Hodge said: "When people have no choice but to contact the revenue to discuss their tax affairs, I find it totally unacceptable that HMRC uses costly 0845 numbers and charges people for the privilege of waiting for the department to pick up.
"As the minutes tick by, the profits of HMRC's phone service provider, Cable and Wireless, rack up as they pocket a proportion of customer call charges."
"Sluggish performance amounted to a taxpayer telephone toll of £136m in lost customer time and call charges in 2011/12," she added. "My concern is that the cost of hanging on the line hits those who call from a pay as you go mobile the hardest."
An HMRC spokesman said the department was "well aware" that in the past it had not delivered the standard of service to which it was committed.
"In 2010/11 we answered 48 per cent of all call attempts, rising to 74 per cent in 2011/12," he said. "By late 2012 we were answering over 90 per cent of calls to our contact centres.
"We are determined to build on this progress and we have invested £34m so we can deliver on our improvement targets earlier than planned."
The spokesman added they have transferred their tax credits phone lines, which accounts for around 40 per cent of calls, from 0845 to 0345 numbers, which are more likely to be covered by call plans.