The bombs that ripped through the Brussels airport and a downtown subway station caused many casualties on Tuesday. One may eventually prove to be Britain's place in the European Union.
The explosions, which struck the city that hosts most of the EU's institutions of government, came almost three months to the day before voters in the United Kingdom will be asked to decide whether their country should remain inside or leave the EU.
London bookmakers raised the odds of a so-called Brexit to 36 per cent, from 33 per cent, on the assumption that the triple bombing in Brussels would make staying in Europe look less appetizing to voters. The pound fell the most among the world's major currencies on Tuesday, sliding more than 1 per cent against the U.S. dollar, to $1.42, on the same speculation.
"Terrorism could break the EU and lead to Brexit," read a headline on the website of the right-of-centre Daily Telegraph newspaper.
A British vote to leave would be a major blow to the 28-member organization. Britain was the second-largest contributor to the EU budget last year.
Before Tuesday's attacks, most of the ferocious debate about Britain's future revolved around the disputed economics of staying or leaving. But within hours of the first blasts at Zaventem airport, Leave campaigners were seizing on the attacks as evidence that Britons would be safer outside the EU.
Prime Minister David Cameron, who wants the United Kingdom to remain in Europe, has argued that Britain is safer as a member of the EU and its joint security institutions such as Europol. That contention is harder to make after the security failures in the EU capital.
"Cameron says we're safer in the EU. Well, I'm in the centre of the EU and it doesn't feel very safe," Mike Hookem, a member of European parliament for the UK Independence Party, tweeted from his office in Brussels just two hours after the first explosions were reported. UKIP was founded in 1993 with the sole purpose of pushing for Britain to leave the EU.
A statement issued by UKIP's press office later quoted Mr. Hookem, the party's defence spokesman, blaming the attacks on the lack of border checks within the 26-country Shengen Area, which Britain is not part of.
"Brussels, de facto capital of the EU, is also the jihadist capital of Europe. And the Remainers dare to say we're safer in the EU!" echoed Allison Pearson, a Daily Telegraph columnist.
Both remarks provoked firestorms of criticism, including accusations of playing politics with the tragedy in Brussels. Former Labour Party leader Ed Miliband, who gave a previously scheduled pro-EU speech on Tuesday, said it was "not the day for either side in the referendum to use this terrible tragedy to make political capital."
Opinion polls ahead of the June 23 referendum already showed the Leave camp gaining momentum in recent weeks, with the most recent poll of polls showing the Remain side ahead by the narrowest of margins, 51 to 49 per cent. In mid-February, the gap was 55 to 45 per cent.
The campaign has divided Mr. Cameron's Conservative Party, with several high-profile members of the cabinet defecting to the Leave campaign in recent weeks.
Gideon Rachman, a columnist for the Financial Times, compared the rising fortunes of the Leave camp with Donald Trump's surprising presidential campaign in the United States.
"When it comes to both Mr. Trump and Brexit, the political establishments in Washington and London find it hard to believe the public will ultimately make a choice that the establishment regards as self-evidently stupid," Mr. Rachman wrote in a column published on Tuesday before the attacks in Brussels.
But, he wrote, the establishment in both countries was reading the political mood wrong. "High levels of immigration and fear of terrorism have increased the temptation to try and pull up the drawbridge and retreat behind national frontiers."