LONDON (AFP) - European Union leaders on Thursday (June 16) warned against Brexit but said the EU would survive if Britain quits, as two new polls showed Britons tending towards a "Leave" vote in next week's referendum.

"I know it's very difficult for us to be optimistic today, we know the latest polls," EU President Donald Tusk said on a visit to Helsinki, amid volatility on the financial markets that has hit the value of the pound.

"The cost will be very high also for us," he said, adding however: "The EU will survive, I have no doubt, it is still much easier to survive when you are 27 member states than completely alone".

At at an economic forum in Russia, European Commission chief Jean-Claude Juncker made a similar point about the implications of a Brexit.

"If Britain is leaving the European Union this will open a period of major uncertainty, both in Britain and in European Union and on a more global level and this should be avoided," Juncker said.

But he added: "I don't think that the European Union will be in danger of death if Britain leaves because we continue the process of closer cooperation in Europe".

If Britain voted to leave the EU, it would be the first country to do so in the bloc's six-decade history.

The warnings came as an Ipsos Mori survey published Thursday showed 53 per cent of voters backing a Brexit compared to 47 per cent who wanted to stay in the European Union.

Another poll by Survation put "Leave" ahead by 52-48, excluding undecided voters.

Four telephone polls in less than a week have now given the "Leave" camp the advantage ahead of the June 23 referendum, confirming a trend that has put European leaders on alert.

Polling expert John Curtice said the race was now too close to call.

"Until this morning I would have said to you that on the balance of probabilities, 'Remain' were the favourites. I think we no longer have a favourite in this referendum," he told BBC television.

In the minutes of its June meeting, the Bank of England warned once again that the outcome of the referendum was "the largest immediate risk facing UK financial markets, and possibly also global financial markets".

Asian and European markets had already fallen following a warning by US Federal Reserve boss Janet Yellen Wednesday that a Brexit could have consequences for global financial markets and the US economic outlook.

London's FTSE 100 share index was down 0.71 per cent to 5,924 points at 1140 GMT, while the pound reached a new two-month low against the euro.

"City watchers are beginning to take the threat seriously and start to price in the possibility of a Brexit," said Joe Rundle, head of trading at ETX Capital.

Overnight, leading business newspaper the Financial Times endorsed the "Remain" camp, saying Britain had benefited from its 43-year membership of the European fold and leaving would "seriously damage" the economy.

It said a choice for "Leave" would favour "a pinched nationalism" and "marginalisation", adding: "A vote to withdraw would be irrevocable, a grievous blow to the post-1945 liberal world order."

Mark Carney, the governor of the Bank of England, is among a raft of economists and senior figures from international organisations such as the IMF to have warned of the economic risks of a Brexit.

But in a letter to The Daily Telegraph newspaper, two former British finance ministers and two former Conservative Party leaders accused him of peddling "scare stories".

"There has been startling dishonesty in the economic debate, with a woeful failure on the part of the Bank of England, the Treasury, and other official sources to present a fair and balanced analysis," wrote Nigel Lawson, Norman Lamont, Michael Howard and Iain Duncan Smith.

They accused Prime Minister David Cameron and finance minister George Osborne of "peddling phoney forecasts and scare stories" to try to "frighten the electorate into voting Remain."

Carney and Osborne will both speak at the City of London financial hub's annual gathering on Thursday evening.

In an article in The Times newspaper, the prime minister accused those backing "Leave" of "playing fast and loose" with readers' financial future "for their political ends".

This will be viewed as a veiled reference to leading Brexit campaigner Boris Johnson, the former mayor of London widely tipped as a likely successor to Cameron.