Britain's Prime Minister, David Cameron and his wife Samantha leave the Central Methodist Hall polling station after casting their votes for the EU Referendum in London, on June 23, 2016. Millions of Britons will vote to stay in or leave the European Union (EU) on Thursday as polling stations across the country opened to the public in the morning. [Photo: Xinhua]

First counting venue Gibraltar declared its result as 19,322 voted to stay in and 823 voted to leave, Gibraltar local authority said late Thursday as the counting is under way across Britain in the historic EU membership referendum.

The estimated turnout for EU referendum is 83.7 percent, while the last general election was 66 percent, Sky News said.

Over 80 pro-Brexit Conservative MPs signed a letter calling for British Prime Minister David Cameron to stay in his post regardless of the referendum result on Thursday night.

The final result is expected to declare at around breakfast time on Friday.