HM Revenue & Customs is to close more than 90 offices, which the PCS union says will lead to 3,400 job cuts.
The union said the closures, across the UK, would be "bad for business, the public and the taxpayer".
However, an HMRC spokeswoman said the union was "scaremongering" and that it had announced no new job losses.
Planned staff cuts to improve efficiency would continue but compulsory redundancies would be avoided wherever possible, she added.
In a written statement, Treasury Minister Stephen Timms said the decision to close offices had not been easy but that staff and unions had been consulted.
"The overriding consideration has to be the department's need to address new and challenging customer demands by restructuring its business and estate in the most effective and efficient way possible," he said.
The offices will close in phases beginning in spring 2009. The PCS says it has official documents that show this will mean job cuts in the following regions:
PCS general secretary Mark Serwotka said: "These closures and job losses will hit businesses, the public and the communities they serve.
'Bitter blow'
"Rural areas will be disproportionately hit, with face-to-face tax advice reduced to a bare minimum and quality jobs taken out of local economies.
"As the recession worsens, this will come as a bitter blow to a dedicated workforce and will lead to a loss of valuable knowledge and expertise."
Mr Serwotka said cuts were already damaging the ability of HMRC to deal with £21.5bn in uncollected tax and the £25bn lost through tax evasion.
But an HMRC spokeswoman said on Thursday: "We have not announced any job cuts today – this is pure scare-mongering."
When HMRC was created by the merger of Inland Revenue and Customs and Excise in April 2005, it had 105,000 staff and two separate office networks with a total of 590 buildings in more than 300 locations.
The spokeswoman said staff numbers had since been cut by 17,000 as part of a "well-publicised strategy to improve HMRC’s efficiency" by 5% each year until 2011.
"So far HMRC has achieved all staff reductions and efficiency targets without compulsory redundancies and it remains our intention to avoid them wherever reasonably possible," she added.
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