As many as seven separate ISIS-linked terrorist plots to attack UK targets have been averted by security forces, the home secretary has said.
Theresa May told the House of Commons on Wednesday afternoon: "In the last 18 months the police and the security services have disrupted seven terrorist plots against the UK.
"All were linked to or inspired by Daesh [ISIS] and its propaganda. We know too that Daesh has a dedicated external operation structure in Syria which is planning mass casualty attacks around the world."
She said that the threat of an international terrorist attack on the UK remains at "severe" – the same level it's been at since August 2014 – meaning an attack is thought to be highly likely.
May added that "continues to be a threat from Northern Ireland-related terrorism", citing the death of 52-year-old prison officer Adrian Ismay, who died from a heart attack in east Belfast after a bomb exploded underneath the van he was in. A dissident republican group claimed responsibility for the bomb.
May pointed out that Brussels is the 14th attack in Europe since the start of 2015, following the Charlie Hebdo shootings; the two people shot at a synagogue and cafe in Copenhagen; the attack which was prevented on French train and the deadly series of attacks in Paris in November last year.
Earlier on Wednesday, the UK government said it was "concerned" about a British man who is missing following Tuesday's terrorist attacks in Brussels, which killed at least 30 people and injured many more, including four Britons.
After David Cameron chaired a second meeting of the government's Cobra emergency committee in response to the attacks, Downing Street said in a statement: "We are concerned about one missing British national and we are in close contact with the Belgian authorities.
"We are aware of four British nationals who were injured in the attacks – three are being treated in hospital, one has already been discharged. Our embassy staff are working to assist all British nationals affected."
The government has yet to name the missing man, but the family of David Dixon, who lives in Brussels but is originally from Hartlepool, have said he has not been seen since the attacks.
Dixon, 53, is thought to have been using the Metro system around the same time a bomb was detonated on a subway train as it pulled away from Maelbeek station in the heart of the city.
His partner, Charlotte Sutcliffe, has searched the city's hospitals for him and is now appealing for information on his whereabouts. The couple have one son.
Her sister, Marie, told Radio 4's Today programme on Wednesday: "It's heartbreaking and very worrying. [We're hoping] that she'll find out from a hospital or through the police that he's safe.
"Understandably she's very, very distressed... Not everybody's been identified yet, it's just [a question of] waiting for that to happen."
Dixon's cousin, Philip Dixon, told the BBC: "Charlotte tried phoning his work and he didn't turn in for work yesterday morning, so we are fearing the worst at the moment."
Friends of Dixon have been appealing on social media for anyone with information to get in touch.
Dixon's employer, Euroclear, said in a statement: "We confirm that David Dixon is a contractor working for Euroclear in Brussels, who remains unaccounted for following yesterday's tragic events.
"We are in touch with his partner and continue to coordinate with the authorities in locating David.
"Our primary concern is the safety and protection of our people, and our thoughts go out to those affected by yesterday's attacks."
The government is no longer advising against travelling to Brussels, but urged people to "remain alert and vigilant, stay away from crowded places, and follow the instructions of the Belgian authorities".
Tuesday saw police forces across the UK step up patrols in key public areas in response to the Brussels attacks and the government confirmed on Wednesday this will continue in the "coming days".
The UK's terror threat alert level remains at "severe" in the wake of the attack, where it's been since August 2014, meaning an attack is highly likely. The government's statement said "the public are advised to be alert but not alarmed".
A statement from the home secretary, Theresa May, on the UK's response to the attacks is expected in the House of Commons on Wednesday afternoon.
Patrick Smith is a senior reporter for BuzzFeed News and is based in London.
Contact Patrick Smith at email@example.com.
Got a confidential tip? Submit it here.