What the terror threat alert levels mean for London, Brussels and Paris

Belgium has raised its terror alert to its highest level across the country

Belgian policemen stand guard in a street during a police action in the Molenbeek-Saint-Jean district in Brussels (Reuters)

Belgium has raised its terror alert to its highest level in the wake of a series of blasts across Brussels’ Airport and a Metro station, killing at least 20 people and injuring dozens.

Two explosions hit Brussels Airport, one of which was carried out by a suicide bomber, Belgium’s federal prosecutor has confirmed. A third explosion occurred at Maelbeek Metro station, close to EU buildings.

Belgium’s interior minister has now raised the country’s entire terrorist threat alert to its highest level, which sees extra soldiers deployed to the streets and security at its ports and borders. All airport operations have been stopped and incoming flights diverted, while public transport has been halted in Brussels and trains and subway stations have been closed.

People in Brussels have been told to stay where they are, bringing the city to a standstill.

Police and intelligence agencies have been on high alert in the UK and France since the terror attacks in Paris in November last year when 130 people were killed.

In the wake of the Brussels attacks, France’s top security official said the country was immediately reinforcing security at airports, train stations and metros.

The threat level in Paris is already high, with the French government warning the public to be extra vigilant in addition to reinforcing its own domestic and security measures.

Threat levels are designed to give the public an indication of the likelihood of a terrorist attack. In the UK the threat level from international terrorism is currently severe, meaning that an attack is highly likely. It is the fourth highest terror threat level set out by the security service.

The UK’s threat levels range from low, for when an attack is unlikely, to moderate, for when an attack is possible but not likely; substantial, for when an attack is a strong possibility; severe, for when an attack is highly likely; and critical, for when an attack is expected imminently.