Plans to close 281 tax inquiry centres risk worsening an already "disgraceful" phone help service, a watchdog warned.

HM Revenue and Customs (HMRC) has been condemned for costing callers £136 million a year through delays in answering calls, with one-quarter of 79 million unanswered. That was despite spending £900m on customer service.

The Commons Public Accounts Committee welcomed planned improvements, including a callback system and a move away from costly 0845 numbers.

But it said there was a "real risk" of things getting worse with new tax and benefit systems likely to add to call volumes as staff numbers were being cut.

HMRC is consulting on closing all of its face-to-face centres in favour of a targeted "mobile" system. It claims the move will save customers almost £12m a year in lost time and travel costs and slash overheads by more than £13m a year.

Margaret Hodge, the Labour MP who chairs the committee, said: "Just how the department is going to improve standards of customer service, given the prospect of its having fewer staff and receiving a higher volume of calls, is open to question."

An HMRC spokesman said: "This report criticises a previous poor standard of service from which HMRC has already recovered.

"To make it cheaper for customers to call us, we 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."