Brussels airport is to remain closed on Wednesday. There will also be no flights on Thursday.
The Foreign Office has updated its travel advice for Belgium, advising against all but essential travel to Brussels.
Eurostar reinstated some services to and from Brussels this afternoon.
The train operator is expecting to run a full service on Wednesday, but warned passengers stations would be busy.
It asked them to check updates and if they are travelling leave at least 60 minutes for check-in.
BA flights to Brussels remain cancelled on Wednesday.
The budget airline has cancelled its flights to and from Brussels today.
There is an increased police presence at Heathrow Airport, and passengers are being advised to check with their flight status with their airline before travel.
Flights are departing and landing as normal from Gatwick, the airport says, but there are increased police and security patrols.
Ryanair said its flights to and from Brussels on Wednesday would operate from Charleroi airport, 46km from Brussels.
Brussels Airlines flights from Manchester have been cancelled, the airline says.
Whilst there are no direct flights to Brussels Airport from Stansted, the airport says that police have increased their presence as a precaution.
A Manchester Airport spokesman, said:
In response to this morning's events at Brussels Airport, the police presence has been increased in land-side areas at Manchester Airport. It is important to note that these are precautionary measures and we continue to closely monitor the situation.
Increased police presence at UK transport hubs
Prime Minister David Cameron said police presence is being stepped up at ports, airports, Tube stations and international railway stations in Britain in the wake of the attacks in Brussels.
In a statement, Assistant Commissioner Mark Rowley, the national lead for Counter Terrorism Policing, said:
As a precaution forces across the UK have increased policing presence at key locations, including transport hubs, to protect the public and provide reassurance.