LONDON (Reuters) - The campaign for Britain to leave the European Union has taken a 4-5 percentage point lead ahead of a June 23 referendum, according to online polls by ICM and YouGov, sending sterling toward three-week lows against the U.S. dollar.
The swing toward "Out" with less than three weeks to go comes as both sides step up their campaigning to try to win over the large number of still undecided voters with warnings over the economy and immigration.
The ICM poll of 1,741 people taken June 3-5 showed 48 percent would vote to leave, up from 47 percent a week earlier, while 43 percent would opt to stay, down 1 percentage point from a week earlier.
The YouGov poll of 3,495 people on June 1-3 showed 45 percent would opt to leave the EU, up from 40 percent in a comparable poll a month earlier, while 41 percent would opt to stay, down from 42 percent.
The poll showed 11 percent of voters were still undecided, down 2 percentage points from a month earlier.
Of the eight most recently published surveys, one opinion poll was tied, two showed In ahead and five have showed Out in the lead, including a TNS online poll published on Monday and two previous ICM polls published last Tuesday.
"There has been a couple of polls in a row all showing movement towards Leave - I suppose there might be finally some movement in the race," Anthony Wells, director of political research at YouGov, said by telephone.
"ICM's weekly tracker and indeed at least two other polls published in the last 24 hours, suggest a move to Leave might have occurred," Martin Boon, director of research at ICM Unlimited, said in a statement.
Sterling fell below $1.44. The pound fell 0.9 percent to $1.4371, having traded at $1.4441 before the latest ICM poll was published. It had hit a three-week low of $1.4352 in early Asian trade.
Wells said the so-called "Purdah" period which has prevented civil servants from making interventions in the referendum campaign since May 27 may have given Out campaigners such as Boris Johnson the space to push their arguments on immigration.
"The assumption is that immigration has sort of cut through from the Leave campaign," he said. "My best guess is that Purdah has allowed Leave to get into the argument and dominate when previously Remain had been dominating and pushing the argument onto the economy."
Immigration dominated last week's campaign after official figures showed net migration into Britain running at its second highest level, despite pledges by Prime Minister David Cameron to bring the numbers down.
Former London mayor and leading Out campaigner Boris Johnson told workers at a warehouse in central England on Monday that the government needed to set out how it planned to deal with continued high levels of EU immigration.
Remaining inside the bloc was the risky option, he said, adding that Britons could face paying billions of pounds to prop up struggling euro zone countries in future.
But speaking at an event in London alongside senior members of rival political parties, Cameron said the Out campaign was promoting "fantasy politics" and needed to set out what a post-Brexit Britain would look like.
"It is time for Vote Leave to come clean about their economic plan for Britain outside Europe, by refusing to set out their vision they are being undemocratic and reckless," he said, adding that Brexit would "put a bomb under our economy".
Finance minister George Osborne, campaigning in Northern Ireland, warned border controls between the province and the Irish Republic were inevitable if Britain left the EU.
A TNS online poll of 1,213 people on May 19-23 showed 43 percent would vote to leave, 41 percent would vote to stay and 16 percent were undecided.
The TNS poll was adjusted for turnout, changing a 3-percentage point lead for In into a 2 percentage point lead for Out.
"Leave supporters are currently more likely to actually turnout on the day which will be critical in determining the outcome," Luke Taylor, head of social and political attitudes at TNS UK, said by email.
"This poll reinforces our view that it is all to play for over the next two and a half weeks."
Additional reporting by Kylie MacLellan; Editing by Alison Williams