Britain is stronger and safer and families are better off in the European Union, the so-called 'in campaign' said on Monday, launching its fight to keep the country in the European Union by branding its opponents "quitters".

Backed by what it called "the best of Britain", the launch of the 'Britain Stronger in Europe' group marks the official start of a battle to win over voters in the referendum on EU membership which is due to be held by the end of 2017.

Debate over Britain's continued membership of the 28-member bloc has dogged Conservative governments for years, with Prime Minister David Cameron conceding ground to Eurosceptics by offering the referendum. But with Britons split over the issue, the vote threatens to cast a shadow over his last five years in power.

Both 'in' and 'out' campaigns are well-financed, attracting multi-millionaire backers, and their opposing arguments are well-honed with many of their proponents having spent years airing them both inside and outside parliament.

Stuart Rose, the 'Britain Stronger in Europe' chairman, Conservative peer and former boss of retailer Marks and Spencer, said the 'in campaign' has attracted broad support, from "men and women, from boardrooms and the shop floor".

"We are entrepreneurs and entertainers. We are students and activists. What unites us is a commitment to make Britain stronger," Rose said, underlining the desire of both sides to show they have appeal beyond the established politics.

"Those who want us to leave Europe would risk our prosperity, threaten our safety and diminish our influence in the world. We know our economy would take a hit, we just don't know how bad it would be," he said in a statement, describing those who wanted to leave the EU as "quitters".

Last week a rival campaign, "Vote Leave", was launched by a group of business leaders and MPs, while those wanting 'Brexit' can also find a voice with another group, the "Leave.EU" campaign, supported by the head of the Eurosceptic United Kingdom Independence Party.

BROAD APPEAL

Both sides are keen to show they involve politicians from both main parties, but also to demonstrate a wider appeal by attracting businessmen and women, and, in the case of the 'in campaign', television celebrities, such as Karren Brady.

Taking aim at the 'out' campaign's argument that they are the true patriots wanting Britain to reclaim its sovereignty, Rose said it was a "very strange kind of patriotism ... to retreat from our position of influence in Europe".

"Would Britain's pride really be enhanced by walking away? Aren't we bigger than that?" he said.

Rose said he supported Cameron's efforts to renegotiate Britain's relationship with the European Union but the country could only do so from within the institution.

Cameron has pledged to try to get "the best of both worlds" from the EU by asking for guarantees that Britain will be kept out of ever-closer union and the euro is not the EU's official currency, protecting the pound.

"As someone who has grafted all my life to build businesses, I know that no one ever changes things for the better by throwing in the towel," Rose said.