[LONDON] Brexit backers moved into the lead in the latest polls on Monday, putting pressure on the pound and Prime Minister David Cameron just over two weeks before Britain's EU referendum.
The WhatUKThinks average put "Leave" at 51 per cent against "Remain" at 49 per cent ahead of the June 23 membership vote that the pro-EU camp says could hit world economic growth if Britain votes to go.
The balance tipped in favour of "Leave" following the three latest opinion polls, which showed a growing number of Britons want their country to become the first to break away from the European Union.
John Curtice, a polling expert at Strathclyde University said the swing was most likely linked to the start of a pre-referendum "purdah" period on May 27 which means government ministries are no longer allowed to take part in the campaign.
The government backs remaining in the EU.
"The government is unable to dominate the media headlines through using the civil service machine," Mr Curtice told AFP.
The trend also comes just days after Brexit campaigners unveiled plans for an Australian-style points-based immigration system if they win.
Matthew Goodwin, professor of politics at the University of Kent, said on Twitter on Monday that "those who argue immigration not central to anti-EU vote don't understand anti-EU vote".
Boris Johnson, the former London mayor and leading Brexit advocate, pressed the immigration message home at a campaign event.
"We have absolutely no control over the people coming from the whole 27 other EU countries, some of them with criminal records," he said.
"We are talking about taking back control of our immigration policy," he added.
The influx of EU workers from countries such as Italy, Poland, Romania and Spain is at near-record highs and anti-EU campaigners say the new arrivals drive down salaries and burden public services.
Mr Cameron, who heads up the "Remain" camp, has emphasised the economic risks of leaving.
He took part in an event Monday with political opponents Labour, the Liberal Democrats and the Greens to accuse "Leave" supporters of "perpetuating an economic con-trick on the British people".
"While they peddle fantasy politics, in the real world our economy is slowing because of the huge uncertainty hanging over Britain's economic future," Mr Cameron said at the "Britain Stronger In" event.
The broadside came after Mr Johnson, a charismatic politician with popular appeal who is seen as the most likely successor to Mr Cameron, warned Britain would pay more by keeping its membership in the EU.
"The risks of 'Remain' are massive," he said.
Financial markets have proved volatile ahead of the vote, particularly so as the referendum campaign heats up.
Federal Reserve Chair Janet Yellen warned of the fallout on markets and the world economy if Britain votes to leave.
"A UK vote to exit the European Union could have significant economic repercussions," she said in a speech on the economic outlook.
The pound fell to 79.05 pence against the euro in Asian trading - its lowest level in three and a half weeks - and stood at around 78.54 pence against the euro at around 1600 GMT.
The pound also fell against the dollar to US$1.4464.
"It is becoming extremely worrying for the financial markets and expect more sterling losses if polls continued to indicate a Brexit lead," said Hussein Sayed, chief market strategist at trader FXTM.
Craig Erlam, senior market analyst at Oanda, said: "With both sides likely to step up their game over the next couple of weeks, I imagine we'll see a lot more volatility in the pound."
Several of the biggest trade unions also urged members to vote to stay in the EU.
The general secretaries of Unite, Unison and the GMB were among 10 union leaders who wrote a letter to The Guardian newspaper claiming that parental leave and holiday rights would be under threat from the Conservative government without EU protection.
"Maternity and paternity rights, equal treatment for full-time, part-time and agency workers and the right to paid leave - continue to underpin and protect working rights for British people," read the letter.
"If Britain leaves the EU, we are in no doubt these protections would be under great threat. The Tories would negotiate our exit and, we believe, would negotiate away our rights," it said.