Nigel Farage, head of the UK Independence Party and a leading voice in favour of leaving the EU, told Sky News he did not expect to be on the winning side.

"It's been an extraordinary referendum campaign, turnout looks to be exceptionally high and looks like Remain will edge it," Farage told Sky News. A spokesman for Farage declined to elaborate.

The survey by pollster YouGov showed Remain ahead by a margin of 52 to 48 percent. Unlike a classic exit poll, it was based on online responses by a pre-selected sample of people rather than a survey of voters as they left polling stations.

Pollster Ipsos-MORI also put Remain in the lead, saying that surveys it had carried out on Wednesday and Thursday gave it a 54-46 margin of victory. An Ipsos-MORI poll published earlier had just a 52-48 split for Remain.

The pound climbed to its highest for this year to $1.4989 , from around $1.4906 just before the poll closed, a move of 0.56 percent. It also firmed against the yen.

The four-month campaign has sharply polarised the nation and the final outcome of the vote could change the face of Europe.

If Britain becomes the first state to leave the EU, the so-called Brexit would be the biggest blow to the 28-nation bloc since its foundation.

The EU would be stripped of its second-biggest economy and one of its two main military powers, and could face calls for similar votes by anti-EU politicians in other countries.

If it votes to stay, Britain has been promised a special status exempting it from any further political integration, but European leaders will still have to address a sharp rise in euroscepticism across the continent.