Information mistakenly released in a major data breach is in the hands of dissident republicans, Northern Ireland’s police chief has said.

The data includes the surname and first initial of 10,000 Police Service of Northern Ireland (PSNI) employees.

It also includes their rank or grade, where they are based and the unit in which they work.

Simon Byrne said the information could be used to “intimidate or target officers and staff”.

“We are working round the clock to assess this risk,” he said.

The chief constable said police were taking measures to mitigate the risk.

He said dissident republican paramilitaries could use the list of names to generate “fear and uncertainty”.

He said that, contrary to some reports, there was no evidence of movement of officers and staff outside the organisation and he paid tribute to the “resilience” of staff.

Mr Byrne said the force was being “strongly supported” by a range of cyber specialists in dealing with the “unprecedented” incident.

He said the PSNI was continuing to liaise with the UK government.

He emphasised that the safety and welfare of officers and staff was his “top priority.”

Mr Byrne said an online service has been set up to deal with any staff concerns.

He said that at the start of Monday there were 45 members of PSNI staff the organisation “hadn’t caught up with” to discuss the breach, adding that things were moving quickly.

The police chief said contact and face-to-face meetings were continuing to be organised and that the details of retired colleagues were not part of the breach.

Liam Kelly, the Chair of the Police Federation for Northern Ireland (PFNI), the body that represents police officers urged all police officers and staff to exercise “maximum vigilance”.

“We must do all we can to frustrate and prevent attacks on our colleagues and their families,” he said

“We have to be strong and determined to do everything we can to minimise risk and that means varying the routes we take to and from work, changing routines and re-assessing our personal security both on and off duty,” he said.

Details released in the breach includes names of people who work in sensitive areas such as surveillance and intelligence.

Simon Byrne has apologised for what he has described as a breach of personal data on an “industrial-scale”.

In March, the terrorist threat level in Northern Ireland was raised from substantial to severe, meaning an attack is highly likely.

It followed an attack on a senior officer who suffered life-changing injuries after being shot several times by dissident republican paramilitaries in February.

Meanwhile, in a separate development, it emerged on Saturday that 200 officers and staff were not informed of the theft of personal data from a superintendent’s car for a month after it was stolen.

A document containing the names of 200 officers and staff was taken along with a police-issue laptop on 6 July.