The Police Service of Northern Ireland (PSNI) earlier apologised after it inadvertently published the information in response to a Freedom of Information (FOI) request on Tuesday.

The breach included the surname, initials, the rank or grade, the work location and departments of all PSNI staff, but did not involve the officers' and civilians' private addresses.

Alliance Party leader Naomi Long said it was a concern that a member of staff, who she understands to be "relatively junior", had access to the sensitive data, adding: "The seriousness of this cannot be underestimated."

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PSNI said its Chief Constable Simon Byrne is cutting his family holiday short to deal with the crisis and he is expected to answer questions from politicians.

Assistant Chief Constable Chris Todd said: "As a service we are acutely aware of the seriousness of this breach and have declared it to be a critical incident.

"We fully understand the very real concerns being felt by our colleagues and their families and we are working hard to do everything we can to mitigate any risk. We are working with our security partners and organisations to investigate this incident."

He said the force has updated personal security advice to all officers and staff and set up "an emergency threat assessment group" to look at the "welfare concerns of our people".

The Assistant Chief Constable added: "As well as general advice on safety and security this multi-disciplinary group will focus on immediate support to those with specific circumstances which they believe place them or their families at immediate risk or increased threat of harm.

"We have also sought the assistance of an independent advisor to conduct an end to end review of our processes in order to understand what happened, how it happened and what we can do immediately to prevent such a breach happening in the future.

"This is an extremely serious situation."

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Wife of officer says family now 'living in fear'

The UK Information Commissioner's office said on Tuesday it was investigating the breach while "working with the PSNI to establish the level of risk and mitigations".

The information, which was available online for up to three hours, revealed members of the organised crime unit, intelligence officers stationed at ports and airports, officers in the surveillance unit and almost 40 PSNI staff based at MI5's headquarters in Holywood, the Belfast Telegraph reported.

PSNI officers have been the targets of republican paramilitaries in recent years and in March the terror threat level in Northern Ireland was raised to severe.

The wife of one serving officer has told Sky News how they are living in fear and told of her anger over how it was allowed to happen.

"We already have to be careful about having that connection with the PSNI and because of that information now being in the public domain we have no control over who knows," she said.

"We also have two young children to protect and there are still people out there who deem police officers and their families as legitimate targets so it just adds that further element of fear to our daily life."