Northern Ireland's terror threat level has been lowered to "substantial" from "severe" for the first time in 12 years, the United Kingdom government announced on Tuesday.

The move by the domestic intelligence agency MI5 follows "significant progress" the province has made toward peace, Northern Ireland Secretary Brandon Lewis said.

"It is a testament to the ongoing commitment to protecting the peace process and tackling Northern Ireland-related terrorism, and the tremendous efforts of the Police Service of Northern Ireland and MI5," he said. "The government remains committed to securing these gains and continuing to make Northern Ireland a safer, more prosperous society for all," he added.

Northern Ireland was plagued by sectarian violence over British rule from the late 1960s into the 1990s in a period known as "The Troubles."

The conflict, which ended with the signing of the Good Friday Agreement in 1998, was characterized by frequent attacks in Northern Ireland and also on the British mainland.

This is the first time the threat level in Northern Ireland has been reduced from "severe" since it was first published in 2010.

The U.K. designates five levels of terrorism threats, from "critical" – the highest, meaning an attack is "highly likely in the near future" – to "low."

"Severe" is the second-highest level, meaning an attack is "highly likely." "Substantial" means an attack is "likely."

The level is set by the Joint Terrorism Analysis Centre and MI5.

Lewis, however, cautioned: "It is not a time for complacency. There is still a minority who wish to cause harm in Northern Ireland.

"As ever, the public should remain vigilant."

Following Britain's departure from the European Union, tensions in Northern Ireland have been stoked by division over arrangements governing post-Brexit trade in the province.

Simmering anger about goods shortages and uncertainty for businesses boiled over into clashes between police and demonstrators last year, reviving memories of the worst days of sectarian violence.

The rest of the U.K. lowered its terror threat level to "substantial" in February.

It had been raised to "severe" after a Conservative deputy was stabbed to death in October 2021, and a failed bomb attack outside a Liverpool hospital a month later.