The taxman has been condemned for delays in answering phone calls that have been costing callers £136 million a year.
The National Audit Office (NAO) said 20 million calls to HM Revenue and Customs (HMRC) hotlines - many of which are 0845 numbers - were not picked up at all last year.
People who did get through were also waiting longer to speak to an adviser - an average of 282 seconds compared to 107 seconds in 2009/10. In the first quarter of this year, some 6.5 million people were left holding on for longer than 10 minutes.
The NAO found that there had been some progress since thousands more staff were drafted in, with the 74% pick-up rate significantly higher than the 48% recorded in 2010/11.
However, the report warned that the figures probably underestimated the issue, as calls are counted as answered even if they do not reach an adviser.
It also criticised the lack of target times for picking up the phone.
The average wait to speak to an adviser after getting connected increased to 282 seconds in 2011/12, but performance is "substantially worse" during busy periods.
Between April and September this year, a quarter of callers - nearly 6.5 million people - waited for more than 10 minutes.
"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 £33 million in call charges while they waited for HMRC to answer the phone and the estimated value of customer time while they waited was £103 million.
"We estimate that if HMRC improved performance to answer 90% of calls and reduced waiting times, it could save customers around £52 million a year. HMRC currently plans to spend £34 million 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.
"Between April and September 2012, 6.5 million customers were left waiting more than 10 minutes before their call was answered.
"Sluggish performance amounted to a taxpayer telephone toll of £136 million in lost customer time and call charges in 2011/12. My concern is that the cost of hanging on the line hits those who call from a Pay-As-You-Go mobile the hardest."
The Labour MP condemned HMRC's targets as "far too soft and way below industry standards".
"In 2011/12, a staggering 20 million calls went unanswered and yet HMRC still managed to exceed its self-set target of answering just 58% of calls," she added.
"I am concerned that customers will continue to receive a substandard service in the years to come as HMRC's 2014/15 targets are less demanding and cover fewer areas than other organisations."
TaxPayers' Alliance chief executive Matthew Sinclair said: "This report exposes a shameful level of service at HMRC.
"Taxpayers will be outraged that HMRC could let 20 million phone calls go unanswered and yet still claim that it is outperforming some arbitrary target.
"It's no wonder people have to call the revenue so often, given that - at 17,000 pages - our tax code is one of the longest and most complicated in the world.
"It is high time that politicians acted to simplify the tax system and make it far less burdensome.
"Not only would this make life easier for ordinary families and businesses, but it would also greatly reduce the workload placed on the taxman."
Shadow Exchequer secretary Catherine McKinnell said: "This report shows the impact and cost to taxpayers of this Government's cuts to HMRC staff which go too far and too fast.
"These costly delays are unacceptable for taxpayers and businesses trying to give HMRC information to ensure they are paying the right amount of tax or receiving the correct level of tax credits.
"With big changes on the way next year, such as real time information and further cuts to tax credits and child benefit, I fear HMRC will struggle to cope.
"Ministers urgently need to get a grip."
An HMRC spokesman said: "In 2010/11 we answered 48% of all call attempts, rising to 74% in 2011/12. By late 2012 we were answering over 90% of calls to our contact centres. We are well aware that in the past we have not delivered the standard of service to which we are committed.
"We are determined to build on this progress and we have invested £34 million so we can deliver on our improvement targets earlier than planned.
"We receive well over 10 million pieces of post every year, and the most recent figures show we are now replying to over 80% within 15 working days."
The spokesman added they have transferred their Tax Credits Phone Lines, which accounts for around 40% of calls, from 0845 to 0345 numbers.