"As a result of this change, we are working closely with the police and other relevant authorities to ensure appropriate security measures are in place," she said in a statement.
The 1998 deal drew a line under decades of shootings and bombings in England carried out by the Provisional Irish Republican Army (IRA), which was responsible for more than half of the 3,600 deaths during the violence designed to pressure the British government into relinquishing Northern Ireland.
However, dissident factions who rejected that deal have continued to operate.
In March, a group known as new IRA carried out an attack in Belfast which seriously injured a prison officer when a bomb exploded under his van.
The last attack on the British mainland occurred in 2001 when there were explosions in London, including a massive car bomb outside the BBC headquarters, and in Birmingham, central England. No one was killed.
Northern Irish police had warned in March that they expected more incidents in the province to coincide with the anti-British 1916 Easter Rising, the most dramatic chapter of Ireland`s independence struggle.
May also said the threat level to the province itself from Northern Ireland related-terrorism remains unchanged at "severe," the second-highest level, meaning an attack is thought to be highly likely.
The threat level to Britain from international terrorism remains unchanged at severe as well, she said.
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