LONDON(REUTERS, AFP) - With two days to go until Britain's referendum on EU membership that will shape the future of the European Union and the West, polls and surveys on Tuesday (June 21) indicated public opinion is so divided that the outcome is too close to call.

Britons vote on Thursday on whether to quit the 28-nation bloc amid warnings from world leaders, investors and companies that a decision to leave would diminish the former imperial power's political and economic influence, unleash turmoil on markets and send shock waves around the Western world.

As each side sought to play its last trump cards, the pro-EU "Britain Stronger in Europe" campaign issued a final poster of a door leading into a dark void with the slogan: "Leave and there's no going back."

And former England soccer captain David Beckham, a hugely popular public figure, added his voice to Remain's list of celebrity supporters. "For our children and their children we should be facing the problems of the world together and not alone," he said.

Leave campaigners stepped up their relentless focus on what they call uncontrolled immigration, saying Prime Minister David Cameron had been warned four years ago his goal of reducing net arrivals was impossible due to EU rules.

The anti-EU UK Independence Party issued a poster showing a traffic jam with the message "The school over-run" and saying nearly one in four of Britain's primary schools were full or oversubscribed.

The EU - already shaken by differences over migration and the future of the euro zone - would lose its second-largest economy, one of its top two military powers and by far its richest financial centre if Britain left the bloc.

George Soros, the billionaire who bet against the pound in 1992, wrote in an article for the Guardian newspaper that a vote to leave would trigger a bigger, more disruptive devaluation than the fall on Black Wednesday, when market pressure forced sterling out of the European Exchange Rate Mechanism.

Meanwhile, Hong Kong tycoon Li Ka Shing, one of Asia's richest men, called on Britain to remain in the EU, warning a vote to leave would have a "detrimental" effect on the entire continent.

There could also be severe political ramifications, with many in the Brexit camp regarding Cameron's leadership as untenable if there is a vote to leave.

Justice Secretary Michael Gove, a senior government figure and one of the leading voices of the Leave camp, said he would"reflect" on his own position depending on the outcome.

KNIFE EDGE

Sterling has been climbing in recent days on the back of more positive opinion polls, suggesting the mood of the public was swinging behind the "In" camp, but even its supporters said the result was on a knife edge.

"Things have improved for the 'Remain' side in last few days but it could still be very close ... I have more confidence over the last few days," junior finance minister Greg Hands told investors at a conference on Tuesday.

An ORB poll for Tuesday's Daily Telegraph newspaper found support for Remain at 53 per cent, up 5 percentage points on the previous one, with support for Leave on 46 per cent, down three points.

"All the signs of ORB's latest and final poll point to a referendum that will truly come down to the wire," said Lynton Crosby, a political strategist who advised the ruling Conservative Party at the last national election in 2015.

The "Leave" camp had "failed to quash the almost ubiquitous perception that it is the riskier of the two options," he said.

Social research body NatCen also published a survey that found Remain on 53 per cent and Leave on 47 per cent, using a method that took on recommendations by an official inquiry into why pollsters got last year's election wrong, although its research was conducted from May 16 to June 12.

However, an online poll by YouGov for The Times showed Leave ahead on 44 per cent, up one point, with Remain on 42 per cent, down two points.

Campaigning was suspended for three days after the murder of pro-EU lawmaker Jo Cox, who was shot and stabbed to death in her constituency in northern England last Thursday.

Some "Leave" campaigners accuse the "Remain" camp of exploiting the death as part of what they portray as a campaign of scaremongering by the establishment at home and abroad.

Turnout is predicted to be key to the result, and the ORB poll found that Remain supporters, who had been regarded as being more apathetic, were becoming increasingly motivated to vote as polling day approaches.

STERLING, SHARES BOOST

Prior to the murder of Cox, polls had shifted towards"Leave", and the indication that an "In" vote was now more likely has boosted sterling.

The pound hit its highest level against the dollar since the start of the year on Tuesday.

According to Betfair betting odds, the implied probability of a British vote to remain in the EU was 75 percent. But analysts in the financial markets were more cautious. "We will go into the vote without high confidence in predicting the outturn in either direction," JPMorgan researcher Malcolm Barr said in a note to clients.

The "Out" campaign says it is the anti-establishment choice, and its message that EU membership has handed political control to Brussels and fuelled mass immigration appears to have struck a chord with many Britons.

That issue again gained prominence when Cameron's former close aide Steve Hilton said civil servants had explicitly told the prime minister four years ago that his target to cut net immigration to the tens of thousands was unachievable because of the free movement demanded by the EU.

"There are good ways of controlling immigration ... but pulling out of the single market, wrecking our economy, that is a bad way," Cameron told ITV's "Lorraine" programme.

The Remain camp has focused monstly on the economic benefits of EU membership and the risks posed by leaving.

Pro-EU leaders, including former prime ministers Tony Blair and John Major, have warned that an exit could also trigger the break-up of the United Kingdom by prompting another Scottish independence vote if England pulled Scotland out of the EU.