Skilled staff are leaving the Office for National Statistics (ONS) ahead of a planned moved from London to Newport in south Wales, union leaders claim.
The relocation plans were first raised in 2004
The FDA union says hundreds of statisticians are taking up other jobs rather than relocate to Newport.
The ONS said "strategies" were in place to retain staff after the move.
Newport Council said it was "never anticipated" that all ONS staff would relocate, and the city had been identified as the "best location".
The relocation of the ONS was first proposed three years ago and aims to save the civil service £75m.
Ro Marsh, national officer of the FDA, said the union supported the relocation of public services outside London, but added: "It is unreasonable to expect more than 400 statisticians to uproot their families and leave their communities, especially when the remuneration at the ONS is below the market rate for their skills.
"Closing the London office completely will therefore require a major recruitment drive for staff with the right combination of skills within a very short period, and the difficulty of this task should not be underestimated."
The warnings come days after the Bank of England told MPs about its concern about the proposed relocation.
"The relocation programme poses serious risks to the maintenance of the quality of macroeconomic data," the bank said.
Rupert Davies from Cardiff praised Newport
"If substantial numbers of ONS staff are unwilling to relocate, the loss of skilled individuals could have a severe impact on a range of statistics."
The Bank of England relies on the ONS for a wide variety of statistics, especially for setting interest rates.
The ONS is the main source of data on topics such as inflation, unemployment, retail sales and economic output.
In response, an ONS spokesman said: "We recognise that relocating and modernising systems is a challenge.
"This brings uncertainty for staff at all levels, but management is all about minimising risk.
"We have in place retention payments for key staff and a range of strategies to help staff with their future careers.
"These are under constant review and we will, if necessary, adjust the pace of relocation to minimise the risk to key statistical outputs."
Newport Council defended the quality of staff the ONS could recruit from the area.
"There is no doubt whatsoever that the ONS will be able to successfully recruit the calibre of staff required to run a highly professional and successful service outside of London in the city of Newport," said a Newport Council spokesman.
"It was never anticipated that all staff would choose to move and therefore not entirely surprising that some of those people would lobby for a change in government policy.
"But to claim it will be impossible to recruit suitable replacements when the service relocates is utter nonsense.
"Newport was chosen as the best location after careful and detailed consideration of demographic and logistical information."
Estate agent Carole Cheshire praised the city
Estate agent Carole Cheshire, who works for Davis and Son in the city, agreed, and said: "Newport is a really up and coming place and it already attracts a lot of people who commute to Bristol and Cardiff because the house prices are a bit cheaper here.
"There's a lot going on as well with the Ryder Cup in 2010 and the redevelopment in the city centre."
Rupert Davies from Cardiff also recommended Newport.
He said: "I often come here. The parking is good and far cheaper than in Cardiff and there are some really nice shops here too."