Plans to close 281 tax inquiry centres risk worsening an already "disgraceful" phone help service, a spending watchdog warned.
HM Revenue and Customs (HMRC) has been condemned for costing callers £136 million a year through delays in answering calls, with a quarter of 79 million calls unanswered. That was despite spending £900 million on customer service.
The Commons Public Accounts Committee welcomed planned improvements, including a call-back system and a move away from expensive 0845 numbers.
But it condemned a "woefully inadequate and unambitious" new target to answer 80% of calls within five minutes which it said was way below industry standards.
And it said there was a "real risk" of things actually getting worse with new tax and benefit systems likely to add to the volume of calls while staff numbers were being cut.
HMRC opened a consultation on closing all of its face-to-face centres in favour of a targeted "mobile" system in homes, businesses or community locations.
It claims the move will save customers almost £12 million a year in lost time and travel costs and slash running costs by more than £13 million a year.
Margaret Hodge, the Labour MP who chairs the committee, said the closures would "undoubtedly put even more pressure on phone lines.
"HMRC considers that it will be able to improve service standards by reducing avoidable contact with customers and using its staff more flexibly," she said. "It may need to put in additional resources, though, to avoid the kind of plummeting performance we have seen in the past."
An HMRC spokesman said: "This report criticises a previous poor standard of service from which HMRC has already recovered.
"In the past three months, HMRC has been answering more than 90% of calls to our contact centres and during the current year we have replied to 84.5% of the 16 million pieces of post we have received within 15 working days.