LONDON • Britain appears to be on course to leave the European Union (EU), with four polls putting the "Leave" campaign ahead of "Remain".

After a series of new polls on Monday put "Leave" ahead, the day's final blow came when The Sun, Britain's biggest-selling newspaper, backed a so-called Brexit on its front page.

"Outside the EU we can become richer, safer and free at long last to forge our own destiny - as America, Canada, Australia, New Zealand and many other great democracies already do," the newspaper said, reported Bloomberg. "If we stay, Britain will be engulfed in a few short years by this relentlessly expanding German- dominated federal state."

The NumberCruncherPolitics estimate of the probability of a Brexit surged to 32.6 per cent from 23.7 per cent. Oddschecker's survey of bookmakers' implied probabilities rose to 42.5 per cent yesterday morning from 33.5 per cent a day earlier. "The momentum is such that it seems inevitable Brexit will be favourite by the weekend," said William Hill spokesman Graham Sharpe.

Until this week, the focus of the "Remain" campaign, with Prime Minister David Cameron at the helm, has been to warn of the economic consequences of a Brexit.

With evidence that his message is failing to cut through, Mr Cameron has turned to the opposition Labour Party to save him. On Monday, his predecessor as prime minister, Mr Gordon Brown, made an impassioned plea to Labour voters not to turn their backs on the EU.

The party's current leader, Mr Jeremy Corbyn, made his own appeal yesterday for voters to back the "Remain" campaign. However, his message may not have been helped by an intervention from Mr Ed Balls, Labour's former Treasury spokesman, who criticised the way the EU handles immigration in an article in The Daily Mirror while also calling for people to vote "Remain".

Polling expert John Curtice, a politics professor at Strathclyde University in Glasgow, said in a Bloomberg TV interview that while it "looks as though the numbers have moved on average a bit towards 'Leave'", the movement is not a large one, and often in referendums, voters move back to the status quo in the final days of the campaign.

The first polls of Monday evening came from ICM, which published both phone and online surveys showing "Leave" opening up a 5 percentage point lead over "Remain".

A telephone poll of 1,000 people conducted from June 10 to 13 found "Leave" at 50 per cent and "Remain" at 45 per cent. An online poll of 2,001 adults conducted over the same period put "Leave" at 49 per cent and "Remain" at 44 per cent.

Phone polls had previously tended to show better results for "Remain", according to Bloomberg.

Then came a YouGov online survey showing "Leave" at 46 per cent with "Remain" at 39 per cent, and an ORB poll putting "Leave" at 49 per cent and "Remain" at 48 per cent among those certain to vote.

But if all voters, even those who might not vote, were taken into consideration, ORB said the motion to stay in the EU would garner 49 per cent of the vote, compared to 44 per cent for the "Leave" camp.

Polling companies were burned by their failure to predict last year's UK general election result, and a year-long inquiry into their problems found the issues would be hard to fix.

Indications are that the referendum is too close to call till the votes are in, given that up to 30 per cent of people will change the way they vote or make up their minds in the week before the vote, with half of these deciding finally only on Polling Day, according to research by academics at the London School of Economics, The Guardian reported.