Nicola Sturgeon could drive unsure English voters towards the European Union exit door, pro-EU sources at Westminster fear.

They have warned the UK Government not to underestimate the impact the SNP leader could have south of the border.

The concern comes as polls suggest that the result of David Cameron’s upcoming In/ Out referendum could be tight.

Survation found that across the UK 53 per cent said that they wanted to leave the EU while 47 per cent wanted to stay.

A source said: “Can you imagine what some English voters will make of Nicola Sturgeon’s strident support for the EU?

“We know that while during the General Election some liked what they saw of the SNP many in middle England, where most of the votes are, were put off by the party.

“The SNP’s stance will not be the biggest factor, obviously, but it could be an issue if the result is close”.

Another said: “The concern is that the strong campaign message from the SNP will be that to stay in the EU is ‘good for Scotland’, which some voters in England will interpret as not good for them."

Recent YouGov polling suggests that 55 per cent of the Scottish electorate want to stay in the EU, compared with 30 per cent who back calls to leave.

However, in England, which accounts for more than 80 per cent of the total UK population, the pollsters found a very different picture.

There 43 per cent supported an EU exit, compared with 40 per cent who wanted to remain.

Experts link the anti-EU feeling in part to a rise in English identity south of the Border.

The UK could leave the EU even if only English voters back that option.

The Prime Minister has rejected the SNP’s call for a “lock” to ensure that voters in all parts of the UK have to agree before there can be any ‘Brexit’.

The Tory leader has also insisted that the UK-wide result will be final.

Professor Roger Scully, of Cardiff University's Wales Governance Centre, who asked YouGov to carry out the poll, predicted that the EU referendum was set to produce an "achingly close" result.

The SNP has said that a so-called ‘Brexit’ could potential trigger another independence referendum.

If Scots vote to stay they would object to being taken out of the EU against their will, the First Minister has warned.

Earlier this week, Mr Cameron himself suggested that the possibility of Scottish independence should be a factor when voters cast their ballot.

In a keynote speech setting out his demands for reform from other European leaders he said that the future of the United Kingdom should be one of the issues voters consider on polling day.

The SNP, meanwhile, is concerned about the impact that Conservatives, which the party argues is still ‘toxic’ in Scotland, will have on the referendum campaign.

Earlier this week the party’s shadow spokesman on Europe Stephen Gethins said that the SNP would create a “distinctively Scottish” campaign in a bid, in part, to avoid being tarnished by the Conservative’s brand.

The SNP argue that Labour made a mistake by joining with the Tories, and their coalition government partners the Liberal Democrats, in the pro-Union Better Together campaign in the run up to the independence referendum.

John McDonnell, the Labour shadow chancellor, has also said that he would not share a platform with the Conservatives during the EU referendum.

An SNP spokesman said: "The SNP will continue to make a clear, strong and positive case for Scotland remaining in the EU - in stark contrast to the weak and confused messages that have been emanating from the Westminster parties.

"With David Cameron recklessly flirting with an EU exit it is more crucial than ever that the focus of all those who support EU membership is on making the case to remain rather than sowing the seeds of division. As today's Survation poll shows a majority of people in Scotland favour staying in the EU. The only people alienating English voters from Europe are the Tories themselves."