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Delays in answering phone calls to HM Revenue and Customs (HMRC) cost callers £136m last year, it has been estimated.

The National Audit Office (NAO) has calculated that members of the public spent £33m on call charges while holding on HMRC phone lines in 2011-12, wasting time worth £103m to them.

Some 20 million calls were not picked up at all last year, the NAO added.

An HMRC spokesman said standards had slipped "in the past" and the body was "determined" to improve.

But Commons Public Accounts Committee chair Margaret Hodge described the situation as "totally unacceptable".

The NAO said callers have been waiting longer to speak to an adviser - an average of 282 seconds compared with 107 seconds in 2009-10.

Many of HMRC's phone lines have 0845 prefixes, which can cost callers on mobile phones between 12p and 41p per minute, whether they are on hold or speaking to someone.

Calls from landlines cost between 1p and 10.5p per minute, depending on the time of day.

In the first six months of the current financial year, some 6.5 million people were kept on hold for longer than 10 minutes - a quarter of all callers.

The NAO found that there had been some progress: 74% of phone calls were answered in 2011-12, compared with 48% in 2010-11.

But it warned these figures "may overestimate the number of answered calls", as calls are counted as answered even when the caller is put through to an automated message and not an adviser.

Ms 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.

She said: "My concern is that the cost of hanging on the line hits those who call from a Pay-As-You-Go mobile the hardest."

"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.

Shadow Treasury minister Catherine McKinnell called on the government to "get a grip" on the situation.

"This report shows the impact and cost to taxpayers of this government's cuts to HMRC staff which go too far and too fast," she said.

"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."

But 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 £34m 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."