The UK’s national terrorism threat level has been downgraded from severe to substantial for the first time in five years, the home secretary has said, although this means a terrorist attack is still likely.

Priti Patel announced the decision by the Joint Terrorism Analysis Centre (Jtac) in a statement to parliament.

The “substantial” threat level continues to indicate a high level of threat and an attack might well occur without further warning, she said.

Changes to the threat level system came into effect in July, meaning the assessment now reflects the threat posed by all forms of terrorism, irrespective of ideology and covers extreme right and leftwing terrorism.

“This is a systematic, comprehensive and rigorous process, based on the very latest intelligence and analysis of internal and external factors which drive the threat,” Patel said. “Despite the change in the threat level, terrorism remains one of the most direct and immediate risks to our national security.”

The threat level under the previous system was raised from substantial to severe in August 2014 against a backdrop of increasing terrorist activity in Syria and Iraq, including the growth of Islamic State. A “severe” threat level means an attack is highly likely.

Counter-terrorism officials said the level of Islamist terrorist activity started to rise dramatically in 2016 and continued on an upward trend into 2017.

Resources were stretched throughout 2017 by a series of plots claiming lives in London and Manchester. Three were incited by Islamist ideology and one motivated by the extreme far right. The threat level was raised twice during the period from severe to critical, meaning an attack could be imminent.

Counter-terrorism officials said the high and enduring tempo of activity has lessened in recent months, but that it was still higher than in 2015. Officials are aware that plots can proceed from planning to execution in a matter of days. Police and MI5 investigators believe they foiled a dangerous terrorist plot in the last month.

The majority of threats come from Islamists, but the far right has been described as the fastest growing threat. Twenty-four plots have been foiled since March 2017, 16 incited by Islamist ideology and eight by far-right ideology.