EU referendum: Brexit campaign ahead for first time as polls near

London: Brexit backers moved into a lead in the polls Monday for the first time in weeks, putting pressure on the pound and raising pressure on Prime Minister David Cameron ahead of Britain's EU referendum.

The WhatUKThinks average now puts "Leave" at 51 percent against "Remain" at 49 percent ahead of the June 23 vote that could see financial markets plunge and hit global economic growth if Britain votes to go.

Read: Brexit, refugee crisis threaten world economic growth, says OECD

The balance tipped in favour of "Leave" after two polls at the weekend and one on Monday put them ahead. The last time it put "Leave" in the lead was on May 12.

The vote could see Britain become the first country to leave the European Union and is being closely watched in countries where euroscepticism is growing.

The "Remain" camp led by Cameron has warned of the economic dangers of leaving, while "Leave" supporters like former London mayor Boris Johnson have said they want to curb immigrants from other parts of Europe.

Cameron took part in an event with political opponents Labour, the Liberal Democrats and 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," Cameron said at the "Britain Stronger In" event.

The broadside came after Johnson, who is seen as the most likely successor to Cameron, warned Britain would have to pay more for membership of the EU.

"The risks of 'Remain' are massive," he said.

Conservative former prime minister John Major on Sunday had accused Johnson, a charismatic politician with popular appeal, of being a "python" who could not be trusted, in a scathing personal attack.

Financial markets have proved volatile ahead of the vote and have been particularly so as the campaign has heated up.

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 78.61 pence against the euro at around 0910 GMT.

The pound also fell against the dollar to $1.4353 -- a three-week low point.

"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 their members to vote to stay in the 28-member 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 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.

In an indication of growing concern, the BBC reported that pro-EU lawmakers were considering ways of using their parliamentary majority to influence negotiations in case Britons vote to leave.

MPs could push for Britain to remain in the EU single market despite losing membership, for example through continued membership of the European Economic Area.

"We would accept the mandate of the people to leave the EU. But everything after that is negotiable and parliament would have its say," the BBC quoted a government minister as saying.

British banks have also begun advising their staff on how to deal with customer queries in case of Brexit, the Financial Times newspaper reported.

It quoted bankers as saying the guidance was to say "there would be no immediate impact, with the aim of reassuring customers and preventing them from panicking".