Two new polls by ICM suggested Britain is on course to quit the European Union, with both phone and online surveys showing the "Leave" side opening up a 5 percentage-point lead over "Remain."

A telephone poll of 1000 people conducted June 10 to 13 found "Leave" at 50 per cent and "Remain" at 45 per cent, ICM said. An online poll of 2001 adults conducted over the same dates put "Leave" at 49 per cent and "Remain" at 44 per cent. Phone polls had previously tended to show better results for "Remain". The findings come in the wake of other polls in recent days that have shown growing momentum for a so-called Brexit.

The campaign to stay in the EU deployed former Labour Prime Minister Gordon Brown today to try to reach his party's voters and persuade them to back continued membership in the June 23 referendum. Chancellor of the Exchequer George Osborne called for pro-EU businesses to speak up about their concerns.

"People who are concerned or businesses who are concerned or investors who are concerned about the prospect of Britain leaving the EU should speak up," Osborne said. "This is not the moment for businesses to sit it out."

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Taking Osborne's advice, Terry Leahy, the former chief executive officer of Tesco, predicted a recession in the event of Brexit.

"Foreign investment will go, jobs will go," he told business leaders in Liverpool. "We cannot afford another recession. This is really serious."

The ICM poll, the latest to show "Leave" pulling away, was hotly anticipated on financial markets. The polling company's website crashed earlier in the day after a rumour spread that it was about to be published. Less than four hours later, the pound briefly spiked when an old ICM poll showing "Remain" 10 percentage points ahead was shared on Twitter before. Martin Boon, ICM's political analyst, pleaded on Twitter for everyone to "calm down a bit".

Brown sought to relaunch Labour's campaign to keep Britain in the 28-nation EU. With Osborne's Conservative Party split over the referendum, "Remain" is hoping that Labour can convince those voters who have yet to decide how to vote to support its campaign as polls suggest the result is on a knife-edge.

"It makes sense to set minimum standards across Europe," Brown said in a speech in Leicester, central England. "Maternity pay, gender equality, holiday pay, a maximum working week - all gained from Europe."

For "Leave," former London Mayor Boris Johnson was dismissive. "If you have a relaunch with Gordon Brown, that's got to be some measure of desperation," he told the BBC.

- Bloomberg