The UK referendum: a timetable

A vote to leave could mortally wound the integration project, which has dominated the European political theatre since WWII.

Even a vote to stay is likely to make Europeans think twice about their common future.

A record number of Britons in the UK, Gibraltar and overseas territories are eligible to take part: 46,499,537 people can head to the ballot boxes, according to early figures published on Tuesday by the British Electoral Commission.

There is no minimum turnout needed for the vote to be valid and binding.

Polls will open at 7AM and close at 10PM local time, when counting gets under way at the UK's 382 local voting centres.

There will be no official exit poll, but media outlets and pollsters are likely to start predicting the outcome as results come out region by region or on the basis of unofficial surveys.

An indicative outcome is expected to emerge at around 4AM.

Final results are expected at around 8AM with British prime minister David Cameron to hold a press conference a few hours after later.

EU reactions

The EU is not planning to comment on the outcome until there are solid results and Cameron has spoken.

But there might be an earlier reaction form Dutch prime minister Mark Rutte, whose country holds the EU's rotating presidency, an EU official said.

The political group leaders in the European Parliament will meet at 8AM Brussels time.

After their meeting, EP president Martin Schulz is to hold a press conference at 9.30AM.

Schulz will then head over to the Berlaymont building, the headquarters of the European Commission.

At 10.30AM Brussels time, the leaders of the EU’s top institutions - commission president Jean-Claude Juncker, EU council chief Donald Tusk, Rutte, and Schulz - will hold talks in the HQ.

They will discuss a joint reaction, with a public statement expected at lunchtime.

Meanwhile, EU affairs ministers will meet in Luxembourg to prepare for next week's EU summit.

Even if Remain wins, the Brussels political elite will be on high alert.

"There will be quite a few phone calls," said one EU official, but he said that no special meetings were planned for the coming weekend, but coordination between EU capitals will be intense.

"If all 27 [other] member states can agree on a single message, that would be quite an achievement in itself," another official said, referring to the difficult consensus-building process among EU countries.

On Monday, EU commissioners will also hold a special meeting, on the eve of the EU summit on Tuesday and Wednesday.