The threat from Northern Ireland-related terrorism has been downgraded from substantial to moderate, meaning an attack is possible but unlikely, the Home Secretary has said.
Theresa May said the threat level to Great Britain is being reduced, but the threat to Northern Ireland remains at severe, meaning an attack there is highly likely.
The threat from international terrorism also remains substantial, meaning an attack is a strong possibility, she added.
In a written statement to MPs, Mrs May said: "Today the Security Service reduced the threat level to Great Britain from Northern Ireland-related terrorism from substantial to moderate.
"This means that a terrorist attack is possible, but not likely.
"The threat level to the UK from international terrorism remains at substantial, which means that an attack is a strong possibility.
"The threat level to Northern Ireland from Northern Ireland-related terrorism remains at severe, meaning that an attack is highly likely. Neither of these two levels has changed.
"Despite the change which has been made today, there remains a real and serious threat against the United Kingdom from terrorism and I would ask the public to remain vigilant and to report any suspicious activity to the police.
"The decision to change this threat level is taken by the Security Service independently of ministers and is based on the very latest intelligence, considering factors such as capability, intent and timescale.
"Threat levels are kept under constant review."
The threat assessment was increased in September 2010 after Jonathan Evans, the director-general of MI5, warned that dissident Irish republicans could attempt a new wave of terrorist attacks on the British mainland.