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.
“”We pledge to support the members affected and will campaign vigorously to reverse this outrageous decision ”
Mark Serwotka General Secretary of PCS
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 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.”
There have already been some cutbacks among civil servants in Wales.
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.”
Staff at Companies House in Cardiff are facing cuts of 11% this financial year with savings of £6m being made but managers have said they believed they could make the savings without compulsory redundancies.
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 1500 people are employed.
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