Simon Byrne is at an emergency meeting of the Northern Ireland Policing Board after cutting short a family holiday.

Earlier this week, the PSNI declared a "critical incident" after releasing information including the surnames, initials, ranks or grades, work locations and departments of all PSNI staff as part of a Freedom of Information (FoI) request.

The second breach involved stolen documents and a laptop.

"This kind of information could be used by terrorists," senior DUP MP Sammy Wilson told Sky News, adding that "serious questions" needed to be asked up to "chief constable level".

Mr Byrne should be considering whether his position is "sustainable", he added.

However, asked whether he was intending to quit, Mr Byrne told the Financial Times: "No, I am not."

Mr Wilson said it was a serious situation. "Terrorists in Northern Ireland are still actively targeting police officers," he commented, adding that they are "always looking for snippets of information that will enable them to identity police officers so that they can then attack them".

Liam Kelly, chairman of the Police Federation for Northern Ireland (PFNI), said he had been "inundated with calls from worried officers".

Michelle O'Neill, Sinn Fein's vice president, said the reasons behind the data breaches needed to be established and that "there needs to be accountability around all of that".

Chris Heaton-Harris, the secretary of state for Northern Ireland, said the initial breach was a "very serious matter".

There are serious concerns about PSNI processes, Mr Wilson said, commenting: "How did this information get through all the filters that should be in place? Or were the filters even there in the first instance?

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"If they weren't, then there need to be serious questions asked about the people at the top of the police service as to why they have not put in place processes that would safeguard the security of their officers.

"Right up to chief constable level questions have to be asked about how come the police in Northern Ireland do not have a process which ensures that information such as this is checked, rechecked, filtered out, before it ever gets made public."

Regarding potential resignations, Mr Wilson said: "In any organisation, the buck stops with the person who heads that organisation. He may not be dealing with the day-to-day operational issues but nevertheless he is the one who sets the policy, and should ensure that that policy is carried out.

"And if it's not carried out, especially in a case like this, I think the chief constable has to ask themselves, 'what role do I play in all of this, and is my position sustainable'?"

Liam Kelly said the PFNI has had "in-depth discussions already with the PSNI senior command and they fully accept and recognise the gravity of this situation and the depth of officer anger and concern".

PSNI Assistant Chief Constable Chris Todd apologised for the initial breach earlier, saying: "I've had to inform the Information Commissioner's office of a significant data breach that we're responsible for.

"This is unacceptable."

He said it was a result of "human error" with the people involved in the process having "acted in good faith".