MI5 has increased the terror threat level in Northern Ireland from “substantial” to “severe”, meaning an attack is highly likely.

Northern Ireland Secretary Chris Heaton-Harris cited a “small number” of individuals who remain determined to use “politically motivated violence”.

Announcing the change on Tuesday, he urged the public to “remain vigilant” but “not be alarmed”.

It comes after a number of incidents targeting security forces in the country, including the attempted murder of Detective Chief Inspector John Caldwell in Omagh last month.

Meanwhile, the threat level for the rest of the UK remains at "substantial". 

Mr Heaton-Harris wrote in a written statement to MPs: “Over the last 25 years, Northern Ireland has transformed into a peaceful society.

“The Belfast (Good Friday) Agreement demonstrates how peaceful and democratic politics improve society,” he said.

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“However, a small number of people remain determined to cause harm to our communities through acts of politically motivated violence.

“In recent months, we have seen an increase in levels of activity relating to Northern Ireland-related terrorism, which has targeted police officers serving their communities and also put at risk the lives of children and other members of the public.

“These attacks have no support, as demonstrated by the reaction to the abhorrent attempted murder of DCI Caldwell.”

Northern Ireland's threat level had been lowered to substantial just a year ago after it was altered for the first time since 2010.

Simon Byrne, Chief Constable of the Police Service of Northern Ireland, said: “This is part of an ongoing process of monitoring the threat level in Northern Ireland, which is conducted by MI5. We have spoken publicly about the number of attacks that have taken place in recent months, not least the attempted murder of DCI John Caldwell on February 22.

“We will relentlessly pursue those who seek to cause harm and terrorise our communities, and attack my officers and staff, and I pay tribute to them as they continue to deliver for our communities.

“I would also like to thank the community and political leaders of Northern Ireland for their overwhelming support for the Police Service in recent times.”