The arrest forms part of a PSNI (Police Service of Northern Ireland) investigation into the alleged theft of sensitive security documents.
It is understood the documents contained information the police and security service MI5 believe could put the lives of individuals at risk.
The documents were released to lawyers without authorisation from the ombudsman, or the PSNI.
The police have suspended the release of any further sensitive material to the ombudsman’s office until a review of security protocols is carried out.
The Police Federation for Northern Ireland said it was an “astounding and very worrying state of affairs” and has called for a full inquiry into the operation of the ombudsman.
A 69-year-old man was arrested in Dartford, Kent, on Sunday after detectives from the PSNI’s Serious Crime Branch conducted a joint search with local police.
It is understood he retired from the ombudsman’s office a number of years ago and that the documents do not refer to any case currently being investigated by it.
In a statement, a spokesman for the ombudsman confirmed he was alerted by police last week about the theft of sensitive material originating from the office.
“The material came to light during legal proceedings not connected to the office,” he said.
“We immediately asked the police to begin a criminal investigation into how and when this happened. We also informed the Information Commissioner.”
Assistant Chief Constable Steve Martin, head of the PSNI’s Crime Operations Branch, confirmed a man had been arrested as part of the investigation.
“The PSNI can confirm that it has become aware of a suspected theft of sensitive documents from within the Office of the Police Ombudsman of Northern Ireland.
“We have now commenced a criminal investigation and are also carrying out an assessment of any impact which may be caused by the unauthorised release of sensitive material.”.
The statement added that the police and Police Ombudsman’s office have agreed to “a temporary period of review during which sensitive information will not be shared.”
In its statement, the office of the Police Ombudsman said it was anticipated that this suspension of the sharing of sensitive material “will be temporary.”
The Police Federation said there had to be an element of fairness for police officers under investigation.
“They need the same protections that a normal member of society would have and that doesn’t exist at the moment with the ombudsman’s office,” said Federation chairman Mark Lindsay.