More than 90 tax offices across the country are to be closed in the last phase of major restructuring, the government has announced.

HM Revenue and Customs published the final stage of its office space review on Thursday, a process begun after the merger of the Inland Revenue and Customs and Excise almost four years ago.

Public and Commercial Services union (PCS) has condemned the move, warning that it would damage public confidence in the service, but their claims that 3,400 jobs would be lost has been branded as “scaremongering” by the department.

Treasury minister Stephen Timms said the decision to close 93 offices had “not been easy”.

But, he added: “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.”

The closures are to start immediately, beginning in the North West, and are not expected to be completed before Spring 2011.

PCS officials have warned that the decision will lead to the loss of more than 3,400 jobs over the next three years at a time when the department was already struggling to collect the £12.5bn worth of uncollected tax after 17,000 posts were already cut since March 2004.

The closures and job losses would also see the department lose valuable skills and experience, the union has warned, and were bound to affect the quality of services.

General secretary Mark Serwotka said the decision, made as recession loomed, would come as a “bitter blow to a dedicated workforce”.

He added: “The government has to recognise that the erosion of public confidence can only be halted by having enough civil and public servants with the right resources to do the job. As the recession deepens and people become more reliant on public services, the department and the government should stop adding to the growing number of unemployed and call a halt to the office closure and job cuts programme.”

But a spokeswoman for HMRC dismissed the union’s claims. “We have not announced any job cuts today – this is pure scare-mongering.”

The 17,000 jobs cut, from an original workforce of 105,000, was part of a “well publicised strategy” to improve HMRC’s efficiency by five per cent every year until 2011, she added. “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.”