A truck is driven by Vote Leave supporters through Parliament Square, ahead of Thursday's EU referendum, in London, Britain June 21, 2016.  [Photo/Agencies]

Meanwhile, the Financial Times' so-called "poll of polls" showed both sides level with 44 percent each.

The Remain camp has been cheered up as British football star David Beckham showed his support and called on people to vote to stay in the EU on Tuesday.

On Tuesday afternoon, British Prime Minister David Cameron also made a speech outside 10 Downing Street to plea for Britons to vote remain.

He stressed that Britain's economy would be stronger if it stayed in the EU, and weaker if it left, saying leaving would make the country harder to keep safe.