The Newport office is expected to close by spring 2012 First Minister Carwyn Jones is to write to David Cameron over the decision to close the passport office in Newport, south Wales.
Mr Jones said the assembly government had been given no warning about the closure which could see 300 staff lose their jobs.
Welsh Secretary Cheryl Gillan has said no final decision has been made and she would lobby the workforce’s case.
But Mr Jones said the government had “completely lost the plot on this one”.
The first minister said he believed there was no economic argument to close the Wales passport office which also serves people in the west of England.
He told BBC Wales: “It’s unbelievable to think that out of seven passport offices there should be nothing in Wales but passport offices in England, Scotland and Northern Ireland.
“They seem to think its acceptable that Wales and the west of England in fact shouldn’t have a service and that people should have to travel to either London or Peterborough.”
He criticised the lack of consultation, saying the announcement had come “completely out of the blue”.
“I want to know first of all when the Wales office knew about this? I’d like to know what the Wales Office plan to do about it? And certainly the workers in the passport office will have my full support in trying to save their jobs,” Mr Jones said.
“Members are willing to fight. They are angry about this cut. ”
Paul McGoay Newport PCS spokesman
“The Newport passport office has been there since 1967, it’s provided a service for people across Wales and indeed outside of Wales. That service will now disappear and the way that this has been handled frankly is unbelievable.”
The Public and Commercial Services Union (PCS) said the loss of 300 jobs at the office by spring 2012 would be “devastating”.
It is holding a meeting on Monday to organise its campaign.
Newport PCS spokesman Paul McGoay said: “Members are willing to fight. They are angry about this cut.
“They believe that this is an attack on the public sector in south Wales and an attack on the people of south Wales in particular.
‘High handed”
The Identity and Passport Service (IPS) said its system currently has too much capacity which could only be resolved by closing a regional passport centre.
It said analysis had found that closing Newport would result in the greatest reduction of spare capacity at the lowest cost to the taxpayer.
Welsh Lib Dem leader Kirsty Williams AM said: “To suggest that every passport office should remain open except for the only one serving south Wales and south west England is at best high handed and will leave millions of people with an inferior service to the rest of the UK, as well as threatening hundreds of jobs.
“Of course, all public agencies must seek to achieve best value for money in very difficult times but this must be done sensitively with the aim of providing a good service across the UK.”
Welsh Secretary Cheryl Gillan said the PCS had been “irresponsible” in the way it had released details of threatened closure before passport office managers had been able to fully inform staff.
She said no final decision has been made on the future of the office and she would lobby the workforce’s case with the home secretary.
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