The threat level posed by terrorism to the UK has been lowered by the security services after “positive developments” in reducing the risk of attacks, officials have said.

It is now set at substantial, meaning an attack is likely, after five years at either severe or critical.

The head of UK counterterror policing said the change “indicates positive developments in reducing the threat from terrorism”.

Assistant Commissioner Neil Basu said officers were still working on about 800 live counterterror investigations, after foiling 24 plots since the Westminster attack in March 2017.

“It is vital that we all maintain a high level of vigilance,” he added. “Police need the continued support of the public and all our partners.”

UK terror plots that were foiled by security services Show all 14 1 /14 UK terror plots that were foiled by security services UK terror plots that were foiled by security services Oxford Street terror plot Lewis Ludlow was jailed for life in March 2019 for planning a 'spectacular multi-victim attack' on Oxford Street in London. He pleaded guilty at the Old Bailey last year to plotting an attack in the UK and funding Isis abroad EPA UK terror plots that were foiled by security services Rosie Cooper MP plot Jack Renshaw admitted plotting to kill his local Labour MP with a machete in June 2018. Renshaw bought a 19in machete intending to use it to kill Rosie Cooper, MP for West Lancashire. His plan was foiled by whistleblower Robbie Mullen, who heard of the plot in a meeting of alleged and convicted National Action members in the Friar Penketh pub in Warrington Hope Not Hate UK terror plots that were foiled by security services 'Poppy terror plot' Nadir Ali Sayed, his cousin Yousaf Shah Syed, and Haseeb Hamayoon, were charged with terrorism offences over an alleged plot to behead a member of public. The trio were arrested in London and High Wycombe, Buckinghamshire on 6 November - three days before Remembrance Sunday in 2014 PA UK terror plots that were foiled by security services National Action arrests Following the arrest of six men in 2018 on suspicion that they were members of the banned far-right terror group National Action, two were jailed. Christopher Lythgoe was found to be leader of the banned group and jailed for eight years, while Matthew Hankinson was jailed for six years. While it was found that Lythgoe did not encourage Jack Renshaw's plot to kill a Labour MP, the judge noted that he "did nothing to stop or discourage" the attack PA UK terror plots that were foiled by security services Shane Fletcher A self-described 'loner', Fletcher planned to kill members of the public at a football match in his home town of Workington. He had been referred to the government's Prevent programme nine moths prior to his arrest after stating that he dreamed about "shooting up a mosque" PA UK terror plots that were foiled by security services Heathrow airport arrests A 19-year-old from Coventry man was arrested at London's Heathrow airport on suspicion of preparing for acts of terrorism in November 2014 UK terror plots that were foiled by security services Extradition of Abu Hamza Radical muslim cleric Abu Hamza was extradited to the US in 2014 after having been jailed in the UK for 7 years for 11 offences under the Terrorism Act Getty UK terror plots that were foiled by security services South East Counter Terrorism Unit arrests Six people were arrested on suspicion of terrorism offences after a series of dawn raids in the south of England in October 2014. Three men and three women were detained separately in two properties in Portsmouth, one in Farnborough and one in Greenwich following an operation by the South East Counter Terrorism Unit. Counter-terror officers said they had disrupted what was believed to be the early stages of what could have turned into a “significant plot” PA UK terror plots that were foiled by security services October 2014 arrests Three men were arrested in central London on 13 October as part of an investigation into Islamist-related terrorism. The arrests come nearly a week after five men were arrested in dawn raids that Whitehall officials said “may have foiled the early stages” of a plan to attack the UK Peter Macdiarmid/Getty UK terror plots that were foiled by security services Anjem Choudary arrest Anjem Choudary, the radical activist and co-founder of the banned al-Muhajiroun group, was arrested in September 2014 as efforts intensify to disrupt the ideological backers of young British Muslims travelling to fight in foreign conflicts. Mr Choudary was among nine men held on suspicion of supporting a banned terrorist group and encouraging terrorism. The arrests came shortly after Mr Choudary fired off a series of angry tweets after David Cameron called on MPs to back air strikes against Islamic State militants in Syria Oli Scarff/Getty UK terror plots that were foiled by security services North West Counter Terrorism Unit funds seizing Police seize £250,000 of cash intended to fund Isis at Manchester Airport and north-west ports. Using powers under the Terrorism Act, the money was confiscated by officers from the North West Counter Terrorism Unit in Ocotber 2014 Getty UK terror plots that were foiled by security services Tarik Hassane arrest A medical student who was offered a place at a London university has been named among four men who are being questioned by counter-terror police after a series of raids across the capital. Tarik Hassane, 21, is believed to have been Tasered when he was arrested on suspicion of being involved in a "significant" Islamist terror plot on 7 October 2015 UK terror plots that were foiled by security services Abu Qatada removed from UK Radical preacher Abu Qatada will not be returning to the UK after being cleared of terror charges in Jordan in 2013 Reuters UK terror plots that were foiled by security services Haider Ahmed knife plot Teenage Isis supporter Haider Ahmed purchased a 15ins hunting knife and threatened to launch a stabbing attack on the Tube. Prosecutors said he may alternatively have been planning to rob someone with the weapon to fund his travel to fight for Isis in Syria He was jailed for preparing an act of terrorism in June 2019 Counter Terrorism Policing South East

Mr Basu appealed for vigilance during the festive season and upcoming general election.

He said: “Officers will continue to monitor the threat locally and respond appropriately. There will not be any change to our levels of commitment when it comes to protecting our communities.”

The changed threat level was announced little over a week after the death of Isis leader Abu Bakr al-Baghdadi in a US raid in Syria.

The head of US Central Command, General Kenneth McKenzie Jr, said the American military suspected the terrorist group’s followers “will try some form of retribution attack”.

“We don’t see a bloodless future because unfortunately this ideology is going to be out there,” he warned last week.

Raffaello Pantucci, the director of international security studies at the Royal United Services Institute (RUSI), said it would take some time for Isis to activate a major plot.

“Any revenge attack might be more of a lone actor attack,” he told The Independent. “Isis has been damaged but the group will persist.”

Isis announced a new leader and spokesman in a propaganda message that called for supporters to “take revenge for their leaders and brothers against the disbelievers and apostates”.

The UK terror threat level is set by the Joint Terrorism Analysis Centre (JTAC), which includes intelligence agencies, police and government departments.

It was last set at substantial in August 2014, before being increased amid an onslaught of Isis propaganda calling for international terror attacks on non-Muslims.

Isis went further than its predecessors by calling for the indiscriminate killing of any “disbelievers” and urging supporters not to ask for permission or advice, sparking a wave of low-tech attacks that caught international security services off-guard.

A report sparked by the 2017 London Bridge attack raised concerns over the way MI5 assesses and monitors potential terrorists last week.

‘He died whimpering and crying’: Trump announces Isis leader Abu Bakr al-Baghdadi killed in US raid in Syria

The chief coroner of England and Wales called for a review of the “imprecise” potential lone actor triage tool after it emerged that ringleader Khuram Butt was initially judged as having “weak capability”.

“Although attack methodologies of low sophistication may often result in lower tallies of dead and injured, they can be harder to detect in the planning and preparation phase,” said Mark Lucraft QC.

“In my opinion, there is a risk that future deaths could occur unless action is taken.”

As well as Islamist-inspired terrorism, the JTAC system was changed in July to include the threat posed by the far right and other groups.

It separately assesses the threat related to Northern Ireland, which has remained unchanged at severe, meaning an attack is highly likely.

In September, Mr Basu named right-wing extremism as the fastest-growing threat to the UK but said it still made up a smaller proportion of work than Islamist-inspired terrorism.

Priti Patel, the home secretary, said a threat review was conducted every six months.

“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,” she said on Monday.

“Despite the change in the threat level, terrorism remains one of the most direct and immediate risks to our national security.

“‘Substantial’ continues to indicate a high level of threat and an attack might well occur without further warning.”