British voters are moving towards voting to leave the European Union in next month's referendum according to two surveys, surprising investors and sending sterling sharply lower.

The "Out" campaign stood ahead of "Remain" in each of the two surveys by polling firm ICM for the Guardian newspaper, one of which was conducted online and the other by telephone.

The split was 52 to 48 points in favour of leaving the EU in both polls if the undecided were excluded.

When the undecided were included the phone poll had 45 per cent in favour of leaving, 42 per cent wanting to remain, with 13 per cent saying they did not know. For the online poll, 47 per cent wanted to leave, 44 per cent to remain, and 9 per cent said they were undecided.

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They were conducted over three days to Monday after official figures showed last Friday that British net migration hit the second highest level on record last year.

Last week, leaders of the Out camp turned their focus back on migration.

Britons will vote on June 23 on whether to remain in the 28-member EU, a choice with far-reaching consequences for politics, the economy, defence and diplomacy in Britain and far beyond.

From US President Barack Obama to the International Monetary Fund, a host of world leaders and international organisations have cautioned British voters about the risks of leaving the bloc it joined in 1973.

The Bank of England has said a British exit, or Brexit, could tip the economy into recession.

Despite the warnings, Out has appeared to gain traction by focusing on the issue of migration.

Many voters are concerned about the strains placed on schools, hospitals and housing from people moving to live in Britain.

ICM said the polls published today gave Out its first lead in one of its telephone surveys.

The sharp swing wrong-footed financial markets, sending sterling to a one-week low against the US dollar.

A poll published on Tuesday in the Daily Telegraph showed support for Out rising but still behind the In campaign.

Lynton Crosby, the Australian political strategist behind Prime Minister David Cameron's election victory, attributed the move to the focus on migration.

- Reuters