There can be few places in continental Europe where people are watching the results of the in-or-out referendum more closely than in Brussels, home to tens of thousands of bureaucrats working in EU headquarters buildings.
Tim Allan, a 28-year-old British national serving as a press attache inside the hulking glass structure of the European Parliament, told AFP he was awaiting the outcome with cautious optimism.
"My plan is to sleep a little until one or two in the morning and then watch it live on TV," Allan said.
He will join friends from the parliament, journalists and others at the "Funky Monkey," a pub just a stone's throw from the European Commission, another giant glass building housing the executive of the 28-nation bloc.
Results are expected as early as 5:00 am (0300 GMT).
Also open all night will be the press centre at the European Council, which hosts increasingly frequent summit gatherings of EU leaders worried about a British departure from the bloc as well as the euro and migration crises.
For Allan, "a part of his identity" is at stake in the referendum.
This confirmed European married to a Frenchwoman may have to apply for his wife's nationality to shield him from the consequences to his career if Britain leaves the bloc.
European Council President Donald Tusk has warned that a British leave vote could lead to the "destruction of not only the EU but also of Western political civilisation."
Things are so tense that mentioning the word, "Brexit," shorthand for a British exit from the EU, has become taboo among bureaucrats in Brussels.
Allan said it has been a rollercoaster ride as bookmaker odds and opinion polls shift, sometimes pointing to a Brexit and other times pointing to Britain's remaining within the union.
Fluctuating world stock markets have only added to the anxiety.
- 'Summer romance' -
The main topic of conversation Thursday at the European Parliament complex was the referendum, along with the Euro 2016 football cup.
"Everybody has been increasing the pressure for weeks," according to a member of the right-of-centre EPP, parliament's biggest group.
"Since the attacks, the atmosphere has been pretty heavy," the EPP member said on condition of anonymity, referring to the Islamic State-claimed attacks that killed 32 people in Brussels on March 22.
And last week's murder in England of pro-EU British lawmaker Jo Cox has "dropped a lead weight," he added.
However, he said the mood in Brussels was less tense than during last year's fraught financial negotiations to keep Greece in the eurozone.
Pedro Lopez de Pablo, a Spaniard who works for the EPP communications office, said most people he knows believe Britain will vote to remain in Europe.
"I tried to organise bets, but everybody is betting on 'remain' winning. So it's not worth it," de Pablo said.
"And even if there is a Brexit, we know the situation will not change," he said in the corridors of parliament.
"The United Kingdom will continue to treat the EU as a summer romance: they promise each other everything and later each one goes his or her own way," he added.
"Frankly, many people who have been here for a long time are just fed up," he said.
As for the flowers at the doorstep of the British mission, they are a little wilted but have otherwise stood up well to the oppressive heat that is threatening to bring powerful thunderstorms.
Watch Sky News' live coverage of the vote, which is being streamed online, below.
a
