"We are concerned about one missing British national and we are in close contact with the Belgium authorities," Number 10 said.
Four British nationals were injured in the attack, three are in hospital, and one has been discharged. The UK Foreign Office has dropped its previous warning against all travel to Belgium but is urging people to remain alert and to stay away from crowded places.
COBRA is an acronym for the Cabinet Office Briefing Room, where the committee meets. Members include the prime minister, senior cabinet ministers, the head of MI5, the police, civil servants and the head of the government department affected.
I have chaired a meeting of Cobra – we are increasing police presence at ports, airports, Tube stations and international railway stations.— David Cameron (@David_Cameron) March 22, 2016
The COBRA team meet in response to crises and can invoke emergency powers including access to property, restricting the movement of people and setting up special courts.
It met following the 9/11 attacks in the US, the 7/7 bombings in London, the January 2015 and November 2015 attacks in Paris and the Tunisian attacks in 2015.
The threat level remains at "severe" in the UK, suggesting a terrorist attack is highly likely. British intelligence officials thwarted seven terror attack plots in the UK in 2015.
In an announcement ahead of Prime Minister's Questions, David Cameron said the UK had increased border controls and screening and extra police had been drafted in.
"The terrorists want to destroy everything our two countries stand for but we will never let them," Mr Cameron said.
The advice for people in Britain from the police is to "run, hide and tell." A video, released by the National Police Chief's Council in December 2015, advises that mobile phones should be put on silent and encourages people to plan for an attack at work or in a public place.