European Commission President Jean-Claude Juncker said on Thursday Britain leaving the European Union would open a period of global uncertainty which should be avoided.

Juncker was speaking at the St Petersburg International Economic Forum in Russia ahead of talks with Russian President Vladimir Putin.

Elsewhere, European Council President Donald Tusk warned that the economic and political consequences of Britain leaving the EU would be dangerous for both Britain and the rest of the Europe.

Britons vote on June 23 on the future of their membership of the European Union, with a poll on Thursday showing 53 per cent would vote to leave the bloc.

"I have no doubt that the direct results of potential Brexit will be really dangerous for our economy, in the UK and the rest of the Europe," Tusk, who chairs EU summits, said in a press conference in Helsinki.

"Political and geopolitical consenquences are completely unpredictable. What you need today is to strengthen our western community."

He also said the latest polls were not encouraging.

"I know it is very difficult for us to be optimistic today, we know the latest polls. But it's still 50-50, everything is possible."

A telephone poll by Ipsos MORI showed 53 per cent of Britons would vote to leave the European Union in a referendum due on June 23, the Evening Standard newspaper said earlier on Thursday.

The survey by Ipsos Mori of 1,257 adults across Britain from June 11-14 showed 51 per cent of all voters wanted to leave the bloc and 49 per cent wanted to stay. But, when filtered for those likely to vote, the poll showed 53 per cent would vote to leave and 47 to remain.

"With a week to go, Leave have outgunned Remain with a series of arguments on immigration and money that are often believed despite being flatly denied by the other side," the chief executive Ben Page told Reuters.

"Polls don't predict but pollsters sometimes do: personally I think that as in Scotland the status quo may triumph at the last minute. But it looks very close," Page said.

Britain's sterling currency fell from $1.4202 to $1.4125 just after the poll was published and is trading at $1.4160, down 0.25 per cent on the day. The probability of an In vote fell 5 percentage points to 60 percent, according to betting odds supplied by Betfair.

It is the first time that the Out campaign has come out ahead in the monthly Ipsos MORI survey since Prime Minister David Cameron pledged in 2013 to hold a referendum, the pollster said.

It also marks a major turnaround since a May poll by Ipsos MORI, which found 37 per cent wanted to leave against 55 per cent who wanted to stay. When weighted for those likely to vote and excluding undecideds, the breakdown then was 60 per cent for In and 40 per cent for out, a spokeswoman for Ipsos Mori said.

The latest June figures were also filtered by those actually registered to vote, and weighted according to a voter's educational background, Ipsos MORI said.

A vote to end Britain's 43-year-old EU membership would spook investors by undermining post-World War Two attempts at European integration and placing a question mark over the future of the United Kingdom and its $2.9 trillion economy.

Such is the concern over a Brexit that the US Federal Reserve cited it as a reason it delayed an interest rate rise this week. Sterling has also fallen, British government bond yields have been pushed to record lows and billions of dollars have been wiped off global stocks.-Reuters