MPs have voiced concern about controversial plans by HM Revenue and Customs to close a number of offices to transform the way it collects tax.
The Public Accounts Committee said it did not believe the reduction from 170 offices to 13 regional hubs in city centres, would save as much money as predicted.
The union representing staff said the Government should now halt the closure plans and properly resource HMRC.
The MPs said they were concerned that HMRC had not thought through all the negative costs to the wider economy of its approach, as well as the impact on local employment.
HMRC is one year into a 10-year plan but the MPs noted it has already changed, setting up a large office in Stratford, east London.
Efficiency savings were estimated to be £300 million by 2025/26 and £80 million a year after that, with 38,000 workers needing to move and 5,000 possibly leaving.
The MPs concluded that HMRC had yet to show it had a "realistic and affordable" plan to deliver such a radical change.
The report said: "We do not believe that it needs to be based in expensive cities across the UK.
"In re-working its business case, HMRC should reconsider whether moving to major city centres is the optimal way to deliver its objectives and achieve value for money."
The scale of the relocation plan carried a "high risk" of disruption to tax collecting and serving customers, said the report.
The MPs called on HMRC to compare the costs and benefits with alternative sites.
Public and Commercial Services union general secretary Mark Serwotka said: "This report confirms what we have been saying since these plans were announced 18 months ago, that closing this many offices poses a significant threat to the operation of HMRC, its service to the public and the working lives of staff.
"Cutting thousands of HMRC staff in recent years has hit the services it provides to the public, yet the department and this Tory Government are ploughing ahead with poorly thought through plans that would mean thousands more job cuts."
An HMRC spokesman said: "We are considering the committee's report and will respond in due course."