Sixteen offices of HM Revenue and Customs are to close across the West Midlands in a move described as devastating by trade unions.
The move is part of a bid to modernise the service, says the Government, but the Public and Commercial Services Union claims the plans "pose a significant threat to the operation of HMRC, its service to the public and the working lives of staff".
It is also calling for the plan to be the subject of a parliamentary review.
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A spokesperson for the HMRC said they would try to "reduce the number of possible redundancies" but also suggested the widespread office closures would "allow staff time to make choices for their future".
HMRC expects between 3,100 and 3,400 full-time equivalent employees to work in the regional centre, based in Birmingham.
Since 2010, more than 10,000 jobs have been cut from the department and 250 offices have closed, plus the network of 281 walk-in tax enquiry centres.
HMRC expects the changes to save the organisation £100 million a year by 2025.
Some of the offices, set to shut down as early as next year, include Bridge House in Brierley Hill, Northgate House in Northampton and Pattison House in Walsall.
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City Centre House in Union Street, Birmingham, will be closed in 2019.
Employees will be moved to an as yet unannounced "regional centre" somewhere in Birmingham while a "Specialist Site" will be opened in Telford.
PCS general secretary Mark Serwotka said: "No-one should be in any doubt that, if implemented, these proposals would be absolutely devastating for HMRC and the people who work there.
"Closing this many offices would pose a significant threat to the operation of HMRC, its service to the public and the working lives of staff and the need for parliamentary scrutiny of the plans is undeniable and urgent."
HMRC's chief executive Lin Homer added: "HMRC has too many expensive, isolated and outdated offices.
"This makes it difficult for us to collaborate, modernise our ways of working and make the changes we need to transform our service to customers and clamp down further on the minority who try to cheat the system.
"The new regional centre in Birmingham will bring our staff together in a more modern and cost-effective building in an area with lower rent.
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"It will also make a big contribution to the economy of the West Midlands providing high-quality, skilled jobs and supporting the Government's commitment to a national recovery that benefits all parts of the 海角视频."
HMRC came under attack earlier this week from the Commons Treasury committee for its "staggeringly bad" record of answering and dealing with calls.
The 16 affected offices in the West Midlands, to close between 2016/17 and 2020/21:
● City Centre House and Norfolk House, in Birmingham, will close in 2019/20 and move into the new regional centre in Birmingham
● Bridge House, in Brierley Hill, will close in 2016/17 and staff will move into City Centre House, Birmingham, before moving to the new regional centre in 2019/20
● Merry Hill, in Brierley Hill, will transfer to Department for Work and Pensions but a timetable has not been scheduled
● Sherbourne House, in Coventry, is set to be closed in 2019/20, with staff moving to the new regional centre
● Princess House, in Northampton, is set to close in 2020/21
● Northgate House, in Northampton is set to close in 2016/17, with staff moving to Princess House in Northampton for a transitional period
● Sapphire East and Royal House, in Solihull, will close in 2019/20, with staff moving to the new regional centre
● Boyd House I and II, Abbey House and Matheson House in Telford will close in 2017/18 but will then form part of the specialist site in Telford
● Parkside Court, in Telford, will close in 2019/20, with staff moving into the new regional centre and some into the specialist site in Telford in 2019/20
● Pattison House, in Walsall, will close in 2016/17, with staff moving to City Centre House in Birmingham as a transitional site. In 2019/20, they will then move into the new regional centre
● Crown House, in Wolverhampton, will close in 2019/20, with staff moving into the new regional centre
● Council Buildings in Worcester will close in 2019/20, staff may move to the new regional centre