The Public and Commercial Services Union (PCS) says the Home Office decision to close the Identity and Passport Service (IPS) Office in Newport, south Wales, will have "devastating consequences" for the local economy.
PCS general secretary Mark Serwotka says the Newport closure will see the loss of around 250 jobs and scores more at IPS offices across the UK are under threat.
He also believes the closure will make it easier to produce fake passports and put national security at risk.
"The Identity and Passport Service is playing fast and loose with the security of the British passport by putting cuts before the needs of the public," said Serwotka.
"Not only will this increase the risk of fraud, the Newport closure represents an attack on an area of the UK economy that is crying out for investment not cuts.
"These closures will lead to mass job losses at a time of rising unemployment, and we will do everything we can to reverse this outrageous decision."
Shadow Welsh secretary Peter Hain also slammed the closure, particularly since first-time applicants must now attend an in-person interview with the IPS to obtain a passport.
"I am extremely angry," he said.
"These job losses will have a huge impact on the local economy, with people in Wales now facing a journey of hundreds of miles to London or Liverpool for face-to-face service at the Passport Office."
"It is frankly an absolute disgrace and a devastating blow to Newport so soon after last week's Ryder Cup success," he added, referring to Newport's recent hosting of the biannual U.S.A versus Europe golf tournament.
"The economy is still fragile, we need support and investment from Government aimed at securing growth -- not these savage cuts to public services being imposed on Wales by the Tories and Lib Dems."
The Home Office has confirmed a "consultation" on the closure of the Newport IPS office is underway but refused to comment on whether there were plans to close other large regional offices located in London, Glasgow, Peterborough, Liverpool and Durham, or any of the extensive network of smaller, tributary offices.
IPS chief executive Sarah Rapson insisted the cuts would "improve efficiency" and that the UK would still deliver a "secure, internationally respected passport."
"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," she said.