Downing Street has said it is “concerned” about missing David Dixon after the Brussels terror attacks.
Four Britons were injured in the attacks, three of whom are being treated in hospital, No 10 said.
David Cameron has chaired a second meeting of the Government’s emergency Cobra committee and will update MPs on the security situation at Prime Minister’s Questions in the Commons.
Home Secretary Theresa May will also make a statement setting out Britain’s response to the atrocity.
The Prime Minister has asked all government departments to observe a minute’s silence at 11am in a mark of respect following the attacks.
Travel advice has been downgraded but Britons in Belgium should remain alert, No 10 said.
Family and friends of missing Mr Dixon have appealed for information on social media after he failed to turn up for work following the attacks and has not been heard of since.
The IT programmer, who lives in Brussels but is originally from Hartlepool, was travelling to work but did not arrive at his office.
A No 10 spokesman said: “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.
“In terms of travel advice, we continue to advise people to follow the advice of the Belgian authorities. Therefore we are no longer advising against travel to Brussels. British nationals in Belgium should remain alert and vigilant, stay away from crowded places, and follow the instructions of the Belgian authorities.
“Here in the UK, we stepped up the security presence at a number of locations across the country yesterday and we will maintain this in the coming days. The national threat level remains at severe (an attack is highly likely) and the public are advised to be ‘alert but not alarmed’.”