Home Secretary Theresa May said the change reflected “the continuing threat from dissident republican activity,” according to UK’s public broadcaster BBC.
The threat level for Northern Ireland-related terrorism is set separately for Northern Ireland and Great Britain — England, Wales and Scotland, MI5 indicated.
MI5 increases terror threat to GB from Northern Ireland to substantial. Home Sec: Terror attack from NI dissidents is a strong possibility.
— Rob McDowall MEA (@robmcd85) 11 мая 2016 г.
In Northern Ireland, the level of terrorist threat remains at “severe,” which means an attack is highly likely.