David Cameron has condemned terror attacks in Brussels as “appalling and savage” as the UK steps up security.
Britain faces “a very real terror threat”, the PM said, announcing extra patrols at ports, airports, Tube stations and major railway stations.
The blasts, which are feared to have killed at least 31 people, hit Brussels Airport and a metro station in the city shortly after 07:00 GMT.
One UK national has been injured in the airport attack, Downing Street said.
The Foreign Office is advising Britons in Brussels to avoid crowds, and has issued an emergency number for those worried a relative may have been affected – 020 7008 0000.
The UK's most senior counter-terrorism officer, Metropolitan Police Assistant Commissioner Mark Rowley, said increased police activity in the UK was a precaution, but not in response to any specific information or intelligence.
In London, he said, the Met was putting more police on duty to carry out patrols at key locations.
The Belgian flag is being flown at half mast above Downing Street.
'Do all we can'
The Belgian government has not confirmed casualty numbers. Belgium's health minister said 11 people were killed at the airport, and the Brussels mayor said at least 20 people were killed at Maelbeek metro station, according to Belgian media.
Prime Minister David Cameron chaired an emergency Cobra meeting to determine the UK's response to the explosions, and said Britain would do everything it could to help the Belgian authorities.
“These are appalling and savage terrorist attacks and I've just spoken to the prime minister of Belgium to give our sympathies and our condolences to the Belgian people,” Mr Cameron said.
“They could just as well be attacks in Britain or in France or Germany, or elsewhere in Europe and we need to stand together against these appalling terrorists and make sure they can never win.”
He said the UK faced a “very real terror threat”, and the UK authorities were continuing to review information coming in – and would raise the terror threat level if there was information of a direct threat.
The UK terror threat level has been at “severe” since August 2014, meaning an attack is highly likely.
Confirming that one Briton was injured, a spokeswoman for the prime minister said: “We don't have confirmation of any other casualties – or, for the worst, fatalities – at this stage but details are still emerging, so that picture and that information could change.”
The attacks come four days after Salah Abdeslam, a key suspect in the Paris attacks, was captured in Brussels.
The Belgian federal prosecutor said it was “probably a suicide bomber” who struck the airport.
Police in the UK have appealed for any UK nationals who were in Brussels and may have images or footage of the incidents to come forward and assist the investigation.
They have set up a website where images and videos can be uploaded.
Meanwhile, Heathrow and Gatwick airports have stepped up security, while flights between the UK and Brussels are disrupted.
Brussels Airport issued a statement confirming all flights were cancelled, with passengers advised to contact their airlines.
The airport is expected to be closed for at least three days, BBC correspondent Gavin Lee said.
In the latest developments:
- Eurostar says no trains are currently running to or from Brussels Midi station. Passengers are being advised to postpone journeys and services are terminating at Lille in northern France
- British Airways says one of its flights from Brussels to Heathrow departed at 07:40 GMT but two later services were cancelled. Two flights making the outbound trip from Heathrow were also cancelled
- The airline says it will make a decision later on flights due to depart this afternoon
- Newcastle International Airport says flight BM1101 from Newcastle to Brussels diverted to Maastricht in the Netherlands
- Flights from Edinburgh Airport to Brussels have been cancelled and passengers are being advised to check with their airline for alternative travel
- The Port of Dover says security checks remain heightened since November's attacks in Paris, with customers urged to leave extra time before travelling
Northern Ireland's First Minister Arlene Foster expressed her shock at the attacks, while Scotland First Minister Nicola Sturgeon said her thoughts were with the people of Belgium. Welsh First Minister Carwyn Jones tweeted: “Deeply concerned with unfolding events in Brussels – thoughts with everyone involved.”