MI5 has raised the official threat level in the UK from Northern Ireland-related terrorism from moderate to substantial.
This means a terror attack on British soil is a "strong possibility".
Home Secretary Theresa May said the move "reflects the continuing threat from Dissident Republican activity".
PSNI Assistant Chief Constable Will Kerr said:
There remains a real and serious threat from dissident republicans in Northern Ireland.
The threat level to the UK from international terrorism remains unchanged at 'severe', which means that an attack is highly likely.
The five threat levels are:
Critical – an attack is expected imminently
Severe – an attack is highly likely
Substantial – an attack is a strong possibility
Moderate – an attack is possible but not likely
Low – an attack is unlikely
Democratic Unionist MP Jeffrey Donaldson expressed surprise at the threat assessment and is seeking an urgent Privy Council briefing to discuss it.
The Lagan Valley MP said: "It is evident that dissident republicans are now active in Great Britain and are examining potential targets. Obviously that's a matter of concern.
"We had no prior indication that the threat level had been increasing. In Northern Ireland, the threat has been severe for some time but quite clearly this is a new development in terms of dissident republican activity."