Around 16 million individuals and businesses calling the taxman for help face hanging on the line for more than five minutes, officials have admitted.
HM Revenue and Customs (HMRC) was attacked for its "low ambitions" after revealing it expects about 20% of the 80 million calls to their hotlines - many of which are 0845 numbers - will take longer than five minutes to answer.
Labour's Margaret Hodge, who chairs the Public Accounts Committee (PAC), said the industry norm was to take eight out of 10 calls within 20 seconds and dismissed the target as a "disappointment".
HMRC was condemned last year for costing callers £136 million a year through delays in answering phone calls.
Chief executive Lin Homer told MPs that cost was made up of the time people lost on the phone to the service and the cost of the call charge, which accounted for about £34 million of the total. "We think if we increase the number of calls handled to the 90% we've held in the last quarter of this year, that will about halve the time of the call," she told the committee.
Ms Homer said that would mean the four minutes, 42 second average waiting time would drop to about two minutes but admitted those timings did not include how long customers had spent dealing with interactive voice recognition systems. Ms Homer said HMRC research showed customers believed a five-minute wait was acceptable. She added: "If we can answer 90% of the calls, which as I say we have been doing for the last quarter, then we think people will wait up to five minutes."
The committee was told from April HMRC would measure call waiting times from picking up the phone to talking to an adviser and expected to have a target of answering 80% within five minutes. Mrs Hodge said: "Eighty per cent in 20 seconds is the industry norm and you're going 80% in five minutes. I think it's a disappointment. It's unambitious."
It follows a report in December by the National Audit Office that said 20 million calls to HMRC hotlines were not picked up at all in 2011. The watchdog also warned that many customers were charged once their call was connected even if they were held in a queue. HMRC insisted it had made "significant" improvements and was also planning to switch to cheaper 03 prefix numbers.
Ms Homer also told the committee that child benefit reforms that will see higher earners stripped of some or all of the payment were going better than planned, but admitted the system could not keep pace with individual changes of circumstance. Labour MP Meg Hillier said she had not received a letter informing her of the impending changes, which require those affected to opt out of payments or face paying the money back through the tax system, although her husband did. Ms Homer said: "One of the issues for us is that our databases aren't always up to date - people move or they will have a different partner."
A HMRC spokesman said: "From April we will start to migrate all our contact centre services from 0845 numbers to those prefixed by 03. This work should be completed by the end of the summer and for the vast majority of our customers means it will cost less to get in touch with us."