Newport passport office is facing the axe
The passport office in Newport is set to close with the loss of 300 jobs.
The Public and Commercial Services Union (PCS) said the move would have a "devastating" impact on the local economy.
There are also fears passport interview offices in Swansea, Wrexham, Newport and Aberystwyth could close.
The Identity and Passport Service (IPS) confirmed it was consulting on closing the Newport office.
The PCS union said: "The closure of the Newport office will leave Wales with a vastly inferior service to the rest of the UK.
"It will devastate the local economy and lead to untold hardship for those who lose their jobs.
"There is no excuse for this - PCS believes that ordinary working people in south Wales should not be made to pay for the crisis and that the government should invest in jobs rather than attack jobs and services."
The passport office at Newport opened in 1967. It is one of seven passport offices in the UK but the only one believed to be closing.
The others are at London, Liverpool, Glasgow, Belfast, Durham and Peterborough.
Newport provides passport facilities to customers from all of Wales and much of the south and south west of England.
There are also unconfirmed plans to close a number of the offices around the UK in which passport interviews are held.
The PCS said closing such offices would "decimate" the service and increase the risk of fraud.
Mark Serwotka, the General Secretary of PCS, added: "We condemn this deplorable announcement.
"We pledge to support the members affected and will campaign vigorously to reverse this outrageous decision."
IPS will still have physical locations in Wales and continue to provide a service in Welsh, including applications in Welsh and correspondence in Welsh.
It said the changes were necessary to reduce the size of the organisation and ensure it is more efficient.
IPS chief executive Sarah Rapson said: "It is never pleasant to implement changes which means jobs are lost, but IPS is taking these steps to ensure it makes the best possible use of taxpayers' money.
"It is with great regret and reluctance that we are consulting on closing our Newport office.
"But by improving efficiency in the passport application processing network, IPS can be smaller and still deliver good customer service and a secure, internationally respected passport without additional funding from the taxpayer."
The IPS said its analysis had found that closing Newport would result in "the greatest reduction of spare capacity at the lowest cost to the taxpayer".
A spokesperson said the service is working with the Wales Office to try to minimise the impact on the region.
A Welsh Assembly Government spokesperson said: "This is extremely disappointing news and we urge the Home Office to reconsider this decision.
"These 250 jobs make a significant contribution to the economy of Newport and south east Wales and this will be a desperately worrying time for the workers and their families."
Welsh Secretary Cheryl Gillan said she was extremely disappointed at "the irresponsible way" the PCS had released details before managers at the passport office had been able to fully inform staff.
She said: "This is a consultation process and no final decision has yet been made on the future of the office.
"I have been lobbying the Home Office on behalf of the workforce in Newport and, indeed, will be raising the matter with the Home Secretary when we meet this evening."
There are more than 30,000 civil servants in Wales. That includes the DVLA in Swansea where more than 5,000 people work and the Office of National Statistics in Newport where 1,500 people are employed.