THE battle over Britain’s membership of the European Union has entered a key phase with the launch of the cross-party Out campaign, Vote Leave, bringing together Eurosceptic business leaders, politicians and academics.

Vote Leave, supported by the three main anti-EU groups - Business for Britain, the Labour Leave campaign and Conservatives for Britain, led by ex-Chancellor Lord Lawson – will seek to be the main Out campaign group.

It has got a head-start on its rival, the In Campaign, which is expected to launch on Monday in London; again, supported by a range of top figures from business, politics and academia.

David Cameron has promised an In-Out poll by the end of 2017 but has been coy about whether or not Cabinet Ministers will be allowed to campaign for either side once he has concluded his negotiations and is ready to put the question to the British public.

The stakes have been raised even higher because First Minister Nicola Sturgeon has made clear that if the UK voted to leave the EU against Scotland’s will, then this would spark a second Scottish independence referendum.

Already, Leave.EU, an Out campaign, has been launched and has received support from, among others, Ukip's Nigel Farage.

At its launch Vote Leave pointed to a new online ICM poll of more than 1900 people taken after the Prime Minister’s Conservative Party conference speech on Wednesday, which found that if he failed in his bid to renegotiate Britain’s relationship with the EU, then, excluding don’t knows, 53 per cent of people would vote to leave the EU while 47 per cent would vote to stay.

“This shows that unless Mr Cameron gets fundamental reform, he could find it difficult to persuade the country to back the ‘remain’ campaign,” said Vote Leave.

However, the poll also showed when people were asked the straight In-Out question without any conditions, then, excluding don’t knows, 53 per cent said they wanted Britain to stay in the EU and 47 per cent said they wanted it to leave.

At the launch of Vote Leave, Kate Hoey, the Labour MP for Vauxhall in London who co-chairs Labour Leave, said: “We must end the supremacy of EU law over UK law. If we vote to leave, then the £350 million we send to Brussels every week can be spent on our priorities like the NHS.

“I want to see a campaign which brings together those from all parts of the UK, who want to take back control of our countries’ laws to the British Parliament.”

John Mills, the millionaire businessman and Labour’s biggest donor, said a No vote in the In-Out referendum would enable Britain to “make our own trade deals and control our own public services”.

Tory MP Steve Baker, who co-chairs Conservatives for Britain, pointed out it backed the Prime Minister in his attempt to renegotiate the UK’s deal with Brussels but that it also supported a cross-party campaign that could fight the referendum if the EU did not agree to Mr Cameron’s terms on reform.

“I personally support the Vote Leave campaign,” declared the Buckinghamshire MP. “I will be voting to leave unless, at the very least, the Prime Minister secures the end of the supremacy of EU law.”

The Out campaign said its treasurers would be Mr Mills alongside Peter Cruddas, the former Tory Party treasurer, and Stuart Wheeler, ex-Ukip treasurer and one of its major donors.

Among its business backers are John Caudwell, co-founder of Phones 4u, Joe Foster, who founded Reebok, Luke Johnson, Chairman of Patisserie Valerie, Lord Kalms, former Chairman of Dixons, and Scots businessman Alastair MacMillan, of White House Products in Renfrewshire.

The Out campaign’s website – voteleavetakecontrol.org - went live at 10pm on Thursday. Vote Leave said it would make an application to the Electoral Commission, the elections watchdog, for designation as the “official Leave campaign when the time comes”.

Meantime, the In Campaign will launch on Monday. A source said: "We are the optimists; we want Britain to be strong, outward-looking, confident and inside the EU."