An influential group of MPs yesterday rounded on Con-Dem government plans to shut every one of Britain's 281 tax enquiry centres.
A new report by Westminster's public accounts committee warned there is a "real risk" that HM Revenue and Customs (HMRC) proposals to close the offices would make an already "disgraceful" service even worse.
HMRC said that its plan to replace counter services with a "mobile" service, which will operate in workplaces and communities, will slash running costs by £13 million a year.
Managers also claim the changes would save the public £12m a year in travel costs and lost work time.
But public accounts committee chair and Labour MP Margaret Hodge said the closures would put more pressure on already overstretched phone lines.
The committee's report revealed 19 million calls to the HMRC went unanswered last year and delays in answering calls cost customers £136m - despite managers spending £900m on customer service improvements.
MPs welcomed plans to introduce a call-back system and a move away from expensive 0845 numbers.
But Ms Hodge warned Con-Dem government welfare reforms will "drive up the number of phone calls."
"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," she added.
Civil servants' union PCS general secretary Mark Serwotka said that closing "all face-to-face tax offices would break the link between people in communities and an essential public service they rely on.
"If, as we fear, flawed research has been used to justify these closure plans then ministers must put an immediate stop to them," he added.
A HMRC spokesman said the "report criticises a previous poor standard of service from which HMRC has already recovered."
And he promised: "We are investing an extra £34m in our contact centres to maintain this industry-standard level of performance."
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