The UK and its allies must work with “greater urgency and joint resolve” to defeat terrorism, Home Secretary Theresa May has said.
She told MPs Britain must do more to support “vulnerable” countries and to counter the “poisonous and repugnant” ideology of so-called Islamic State.
It comes after bomb attacks in Brussels left 31 people dead. Some 260 people were injured, including four Britons. Another Briton, David Dixon, from Nottingham, is missing. The IT programmer, who lives in the Belgian capital with his partner and son, went to work on the metro, but his family have not heard from him since.
Twin blasts hit the city's Zaventem airport at about 07:00 GMT on Tuesday. Another explosion at Maelbeek metro station near EU headquarters happened an hour later.
Two of the men who carried out suicide attacks have been named as the brothers Khalid and Brahim el-Bakraoui. IS has claimed it was behind the bombings.
Eleven people died at Brussels airport and 20 at the Maelbeek metro.
Of the injured Britons, three, all men, remain in hospital, while the fourth, a woman, has been discharged.
Mrs May said the government was concerned about Mr Dixon and consular staff were helping all British people affected.
Updating the House of Commons following the attacks, the home secretary said:
- There have been 14 plots by IS – also known as Isis, Isil or Daesh – since the start of 2014
- Seven plots have been foiled in the UK in the past year and more than 20 passports seized from British terror suspects to prevent them travelling to join IS
- Following events in Brussels, the UK threat level remains at “severe”, meaning an attack is highly likely
- Security has been stepped up at ports, airports and railway stations, and more Border Force staff deployed to carry out checks on people and vehicles arriving in the UK
Mrs May said the decision to tighten security was not “in response to specific intelligence”, but she said the UK must remain “vigilant”.
“The police and security services will continue in their dedication to keeping people safe and the public should remain alert.”
She continued: “Together with our allies around the world we must act with greater urgency and joint resolve than we have before.”
Shadow home secretary Andy Burnham said: “This is the moment not for division but for maximum unity amongst peoples of all faiths and none.”
He raised concerns about resources for UK Border Force, which he said had endured “years of cuts and is already stretched to the limit”, and questioned whether police outside London had sufficient firearms capability.
Earlier, Lord Reid, who was Labour home secretary until shortly before the 7 July 2005 bombings in London, told the BBC's Victoria Derbyshire programme a terror attack in the UK like that in Brussels was inevitable.
“Politicians ought to be honest with the British people and tell them, 'This will happen.'
“It will happen here because the terrorists only have to get through once,” he said.
The Foreign Office is warning Britons travelling to Brussels to be “alert and vigilant and stay away from crowded places”.
Zaventem airport will remain closed until at least Friday and flights between there and the UK are disrupted. Passengers are being advised to contact their travel operator.
Eurostar services are operating normally on Wednesday, but passengers are being advised to check.
The Port of Dover says security checks have remained heightened since November's attacks in Paris, with customers urged to leave extra time before travelling
An emergency number for those worried a relative may have been affected has also been issued – 020 7008 0000.
More about the attacks