Police services in London, New York and Washington have stepped up security measures today, in the wake of two apparently coordinated attacks in Brussels this morning.
Around 200 flights have been diverted from Zaventem Airport and Eurostar services into Belgium have been suspended, and major cities around the world have been put on high alert as a result.
Speaking early afternoon, David Cameron confirmed police presence had been increased at the UK's airports, ports and the Tube.
"We will never let them win," he said.
Mark Rowley, the national lead for the Metropolitan Police's counter terrorism arm, confirmed the UK had increased policing at "key locations including transport hubs" as a precaution, though insisted it was not as a result of specific intelligence.
Additional officers have been mobilised throughout London to carry out "highly visible patrols at key locations around the capital including the transport network". The officers were being deployed as part of "reassurance measures", he added.
“We are in close liaison with the Belgium authorities and will continue to monitor the situation. The threat to the UK from international terrorism remains at severe as it has been since August 2014, meaning an attack is highly likely."
The government has made a special phone line available for people to contact if they are concerned about friends or family: 0207 008 000.
In the US, at least two cities - Washington and New York City - have stepped up security. Throughout Europe, security has been increased at airports in Paris, Prague, Amsterdam, Vienna, and many others.
In Greece, police added additional security at airports, metro stations and embassies with uniformed and plain-clothed officers.
In Moscow, Russian transport Minister Maxim Sokolov has told Russian news agencies that authorities will "re-evaluate security" at Russian airports.