The incident on Thursday follows two recent breaches of data which put the details of police staff in Northern Ireland at risk.

Officers say the laptop was deactivated and has been recovered, but sections of the notebook remain '"outstanding".

The first data breach saw the details of 10,000 people working for the Police Service of Northern Ireland (PSNI) leaked accidentally following a Freedom of Information request.

Another in July related to the loss of documents and a laptop stolen from a car.

Responding to the latest incident, Assistant Chief Constable Chris Todd said: "It is believed the material fell from a moving vehicle on the foreshore stretch of the M2 motorway, on Thursday 17 August around 4.15pm.

"The laptop was immediately deactivated and has been recovered.

"Sections of the notebook remain outstanding."

ACC Todd added: "Inquiries are under way to establish the contents of the notebook.

"We will be liaising with the Office of the Information Commissioner and have advised the Northern Ireland Policing Board and the Department of Justice.

"Anyone with information or who recovers any material is asked to contact police on 101."

Arrest after first breach

Detectives investigating the first breach have arrested a man on suspicion of collection of information likely to be of use to terrorists.

The 39-year-old was arrested following a search in Lurgan, Co Armagh and is being questioned by detectives at Musgrave serious crimes unit in Belfast.

Detective Chief Superintendent Andy Hill said: "We will continue in our efforts to disrupt criminal activity associated with this Freedom of Information data breach and to keep communities, and our officers and staff who serve them, safe."

The force declared a critical incident earlier this month after data on all 10,000 officers and staff was shared online in response to a Freedom of Information request.

Details released included the surname and first initial of every employee, their rank or grade, where they are based for work and the unit they work in.

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PSNI Chief Constable Simon Byrne confirmed the information is now in the hands of dissident republican.

It followed the posting of documents from the leak on a wall near a Sinn Fein office in Belfast.

The party said a photo of its policing spokesperson Gerry Kelly and a threatening message were posted alongside a redacted version of the documents.

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

Earlier this year, dissident republicans were blamed for the attempted murder of senior PSNI detective John Caldwell.