London (AFP) - Supporters of Brexit moved ahead in the latest opinion 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 of the six last polls put "Leave" at 51 percent against "Remain" at 49 percent ahead of the membership vote on June 23 that the pro-EU camp says could hit world economic growth if Britain votes to quit the bloc.
The balance tipped in favour of "Leave" following the three latest polls, which showed a growing number of Britons want their country to become the first in history to break away from the European Union.
John Curtice of Strathclyde University, seen as Britain's top polling expert, said the swing was likely linked to the start of a pre-referendum "purdah" period on May 27.
This means government ministries are no longer allowed to take part in the campaign. Cameron's government backs remaining in the EU.
"The government is unable to dominate the media headlines through using the civil service machine," Curtice told AFP.
Brexit campaigners have also captured headlines in recent days with plans for an Australian-style points-based immigration system.
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".
- Immigration, economy key -
Playing on the immigration theme Monday, Boris Johnson, the charismatic former London mayor and leading Brexit advocate, told a campaign event: "The risks of 'Remain' are massive".
"We have absolutely no control over the people coming from the whole 27 other EU countries, some of them with criminal records," Johnson said.
"We are talking about taking back control of our immigration policy."
The influx of EU workers from countries such as Poland and Romania is at near-record highs and anti-EU campaigners say the new arrivals drive down salaries and burden public services.
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, warning that leaving the EU would "put a bomb under our economy".
"While they (the 'Leave' camp) peddle fantasy politics, in the real world our economy is slowing because of the huge uncertainty hanging over Britain's economic future," Cameron said.
Financial markets have proved volatile ahead of the vote, particularly as the referendum campaign heats up.
The pound fell to 79.05 pence against the euro in Asian trading -- its lowest level in three and a half weeks -- and was also down against the dollar at $1.4454 around 1930 GMT.
"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.
US Federal Reserve Chair Janet Yellen warned of the fallout for the 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.
- Unions for 'Remain' -
NATO Secretary General Jens Stoltenberg also warned that both the military alliance and the EU were stronger with Britain as part of them.
"We live at a time of many threats and instability, we don't need more instability, we need more cooperation in Europe," he said in Brussels.
Several of the biggest trade unions in Britain 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."