The lives of thousands of retired soldiers in Northern Ireland could have been put at risk after marked letters were sent to their homes.
The Ministry of Defence has apologised “unreservedly” for the security blunder which could have put the lives of 3,000 veterans in danger.
Letters in marked envelopes containing pension information were sent out to former services personnel.
The envelopes were clearly marked and identified the recipients as ex-military staff and sent on behalf of the Service Personnel and Veterans Agency.
Its logo was prominently displayed and the letters were stamped for ex-service personnel.
About 3,000 retired armed forces personnel in Northern Ireland have now been told their security may have been compromised.
The threat level from dissident republicans is currently classed as “severe”.
Ulster Unionist Party MLA Basil McCrea, said he was “staggered” that such an obvious mistake could be made.
“There is obviously a risk and you wonder how something like this could happen,” he said. “It simply beggars belief how people dealing with veterans could not recognise the danger.
“It shows a startling level of incompetence within an organisation which should know better.”
Mr McCrea also said he hoped no one would be harmed as a result of the mistake.
“The damage has been done and I can only hope no harm comes to those who have served the country so well in the past,” the assembly man said.
The Minister of Defence apologised to those involved and said it is investigating how the mistake happened.
An MoD spokesman said: “We would like to apologise unreservedly for identifying our Armed Forces veterans in this way. This simply should not have happened and we have already alerted and apologised to all those involved.
“An urgent security investigation is now under way. Anyone concerned should contact the Veterans Helpline on 0800 169 2277.”