The UK’s terrorism threat level has been downgraded from «severe» to «substantial», the Home Office says.

Home Secretary Priti Patel said the UK is still at «a high level of threat» and an attack could «occur without further warning».

The terrorism threat is now at its lowest since August 2014. Substantial is the third of five ratings at which the threat level can stand. The separate terrorism threat level for Northern Ireland remains «severe».

Ms Patel said in a statement on Monday that terrorism remains a «direct and immediate» risk to the UK’s national security.

Assessments determining the country’s threat level are taken by the Joint Terrorism Analysis Centre (JTAC) — part of MI5 — which makes its recommendations independent from the government. «Government, police and intelligence agencies will continue to work tirelessly to address the threat posed by terrorism in all its forms,» Ms Patel said.

The threat level is kept under «constant review», she added.

Neil Basu, head of counter terrorism policing, said there had been «positive developments» in the fight against terrorism but it was «vital that we all maintain a high level of vigilance». He said the UK’s counter terrorism policing team had about 800 live counter terrorism investigations — while 24 attack plots had been thwarted since the Westminster attack in March 2017.

The UK’s terrorism threat level was raised to the highest rating, «critical», in May 2017 after the Manchester Arena bombing. It was downgraded to «severe» in September 2017 and remained at that level until Monday.

The Northern Ireland threat level specifically refers to threat to the country from Northern Ireland-related terrorism. It remains at severe — the second-highest level.

The five levels of threat set by the JTAC are:

Low — an attack is highly unlikely
Moderate — an attack is possible but not likely
Substantial — an attack is likely
Severe — an attack is highly likely
Critical — an attack is highly likely in the near future