A "disgraceful" tax helpline run by HM Revenue and Customs (HMRC) costs callers £136m a year through delays in answering calls, a spending watchdog has said.
The Commons Public Accounts Committee found a quarter of the 79 million annual calls to the HMRC phone line go unanswered, despite a £900m investment in customer service.
It said a new target to answer 80% of calls within five minutes was "woefully inadequate and unambitious" and questioned whether waiting times would increase if staff numbers are cut.
HMRC plans to close 281 tax inquiry centres in favour of a targeted "mobile" system in homes, businesses or community locations, claiming the move will save customers £12m a year in lost time and travel costs.
But Margaret Hodge, the Labour MP who chairs the committee, said changes to the way companies report payments to employees, as well as a shake-up of tax and benefit systems, would "drive up the number of phone calls to the department".
"Just how the department is going to improve standards of customer service, given the prospect of it having fewer staff and receiving a higher volume of calls, is open to question," she said, adding that HMRC should be aiming to answer the majority of calls within 20 seconds.
A spokesman for HMRC said it had "already recovered" from its "previous poor standard of service".
"In the past three months, we have been answering more than 90% of calls to our contact centres," he said.
"During the current year we have replied to 84.5% of the 16 million pieces of post we have received within 15 working days.
"We are investing an extra £34m in our contact centres to maintain this industry-standard level of performance.
"To make it cheaper for customers to call us, we have already transferred our Tax Credits phone lines from 0845 to 0345 numbers, and will begin to move our remaining lines to 03 numbers from April.
"We will continue to build on these improvements until we deliver the consistent quality of service that our customers are entitled to expect."