Dissident republicans claim to be in possession of information about police officers circulating on WhatsApp following a data blunder, Northern Ireland’s Chief Constable Simon Byrne has said.

Hundreds of police officers have voiced fears for their safety following the data breach which affected some 10,000 Police Service of Northern Ireland (PSNI) officers and staff.

Speaking following an emergency meeting of the Northern Ireland Policing Board on Thursday, Mr Byrne said he was “deeply sorry” about an “industrial scale breach of data”.

He said: “An early worst case scenario that we have been dealing with is that third parties would attempt to get this data to intimidate, corrupt or indeed cause harm to our officers and staff.

“We are now aware that dissident republicans claim to be in possession of some of this information circulating on WhatsApp, and as we speak we are advising officers and staff about how to deal with that and any further risk that they face.”

He added that the force has not yet been able to verify the substance of the claim and the priority “has to be remaining alert to the safety and welfare of both officers and staff as we deal with this unprecedented incident”.

Mr Byrne was questioned for several hours by Policing Board members.

Speaking after the meeting, the board chair Deirdre Toner described the situation as “a very grave matter”.

She said it “will remain the focal point for board meetings with PSNI for many months to come until we are reassured that the recommendations from the review are fully implemented”.

Up to 40 officers at MI5’s headquarters in Co Down are reportedly among the names involved in the blunder earlier this week, with moves under way to ensure their protection.

The incident happened when the PSNI responded to a Freedom of Information request seeking the number of officers and staff of all ranks and grades across the organisation.

In the published response to this request a table was embedded which contained the rank and grade data, but also included detailed information that attached the surname, initial, location and departments for all PSNI employees.

Details of another breach following the theft of documents and a laptop from a car in Newtownabbey in July emerged on Wednesday.

Mr Byrne cut short a family holiday to return to Belfast to be questioned by politicians at the Policing Board meeting, which was held in private.

He said he would not be stepping down, adding: “In the short term my priority is about the wellbeing of officers and staff as we navigate our way through this crisis.

“But equally I know it’s a question that people will be asking, I don’t think leadership is about walking away, it’s facing up to your responsibilities and I think the organisation needs consistency and calm heads at the moment across the team to lead us through what we accept is an unprecedented crisis.”

Asked if members of the Policing Board mentioned him resigning, he replied: “No, they didn’t.”

The Police Federation for Northern Ireland (PFNI), which represents rank-and-file officers, said on Wednesday that they had been inundated with calls from worried officers following the blunder.

Mr Byrne said no police officers have been moved from their home.

Police in the region are under threat from terrorists, with the current assessed level of threat at severe, meaning an attack is highly likely.

In February, senior detective John Caldwell was seriously injured when he was shot by gunmen at a sports complex in Co Tyrone.

Earlier this year, Mr Byrne said he received briefings almost every day about plots to attack and kill his officers, adding that the threat from dissident republicans remained a “real worry”.

Speaking after the meeting, Ulster Unionist MLA Mike Nesbitt, who sits on the Policing Board, said he hopes the dissident republican claim to have the information is false.

“Clearly it is in the interests of dissident republicans to make the claim because it’s in their interests to spread maximum panic, but if they do have it then the police are, I would suggest, going to be well stretched to protect that number of people,” he said.

Mr Nesbitt said Mr Byrne’s position was not discussed at the meeting.

“I’m not going to give a kneejerk reaction to it, and my focus is on the security and the safety of the men and women of the police service and of the service team, and I think Simon is genuine about doing all he can to deal with their safety and security,” he said.