Britain’s security arrangements and counter-terrorism effort will come under scrutiny again following the Brussels attacks.
Police and intelligence agencies have been on high alert since the atrocity in Paris in November in which 130 people were killed.
Authorities have foiled seven plots to attack the UK in the last 18 months and the official threat level is at “severe”.
This is the second highest category and indicates that a strike is seen as “highly likely”. The level was raised from “substantial” in August 2014 in response to the increasing influence of Islamic State in Syria and Iraq.
Since the start of the conflict in Syria, more than 800 people from the UK who are “of national security concern” are thought to have travelled to the region and authorities believe that around half have returned.
An urgent security review was launched in the wake of the massacre in the French capital.
Counter-terrorism services have been braced for a roaming, mass casualty attack since the Mumbai outrage in 2008.
Last summer, police tested their response to such an incident by carrying out a mock exercise in London which saw officers and emergency services respond to reports of shots fired by a group of men who disappeared into the underground network.
In January, Scotland Yard confirmed that its contingent of armed officers will rise by more than a quarter.
Fresh warnings about the enduring threat from terrorism have been issued in recent weeks.
In January, a report by the EU’s law enforcement agency Europol warned that Islamic State is believed to have set up a specialist command as it plots fresh Paris-style attacks.
Earlier this month, Assistant Commissioner Mark Rowley, Britain’s most senior counter-terrorism officer, warned that the group “has big ambitions for enormous and spectacular attacks”.
In the immediate aftermath of the attacks in the Belgian capital on Tuesday, people in the UK may notice an increase in police personnel at airports and stations.
Forces confirmed they were stepping up their presences at key locations including transport hubs. However, officers stressed this was a precautionary move and not based on any specific information and intelligence.
Religious and political leaders around the world have spoken of their horror and outrage at the terror attacks in Belgium.
This is how leaders reacted to the atrocities:
• Belgian prime minister Charles Michel said: “What we feared has happened, we were hit by blind attacks”.
• The Archbishop of Canterbury Justin Welby said: “In the great Holy Week of Christian prayer and mercy, the Brussels attacks shock all those who seek peace and justice through the terrible cruelty and utter separation from all that is of God.
“Once again we see the contrast between the vain efforts to terrify through indiscriminate murder, and the call of God to be those who show mercy, who seek peace and pursue it. Let us at every service this week pray for those caught up in the traumatic events at the airport and in the city of Brussels.”
Nato Secretary General Jens Stoltenberg said: “I am deeply saddened by the attacks here in Brussels this morning. The heavy human toll is tragic.
“My thoughts are with those who have lost their loved ones, with all those affected, and with the people of Belgium. We all stand together with our ally Belgium on this dark day.
“This is a cowardly attack. An attack on our values and on our open societies. Terrorism will not defeat democracy and take away our freedoms.”
• Prime Minister David Cameron said: “I am shocked and concerned by the events in Brussels. We will do everything we can to help.”
• Leader of the Labour Party Jeremy Corbyn said: “I am extremely concerned for all caught up in unfolding events in Brussels. Solidarity with emergency services & those affected.”
• French president Francois Hollande said: “Terrorists have hit Belgium, but it is Europe who has been divided, and it is everyone who has been concerned.”
He added: “A war against terrorism is now being fought by all of Europe, and this is a war that is necessary....this war against terrorism needs to be carried out with cold blood (composure).”
• A statement on behalf of King Philippe and Queen Mathilde of Belgium said: “The King and Queen are shocked by the attacks at the airport and on the Brussels metro. These acts are odious and cowardly.
“The heartfelt thoughts of the King and the Queen are firstly with the victims and their families and the emergency services which are making every effort to help the victims.”
• Belgium and Manchester City football captain Vincent Kompany, who grew up in Molenbeek, tweeted: “Horrified and revolted. Innocent people paying the price again. My thoughts are with the families of the victims. #Brussels
“I wish for Brussels to act with dignity. We are all hurting, yet we must reject hate and its preachers. As hard as it may be. #Brussels”