The "Remain" pro-European group launched its referendum campaign on 12 October to keep Britain in the European Union (EU), cautioning that turning away from the world's biggest economic bloc would hurt jobs and diminish London's global financial clout.

"The choice facing us in this referendum is the biggest choice that we have had for perhaps a generation...Do we continue to lead the world by leading in Europe or risk diminishing our influence on the world stage by turning our backs on Europe?" veteran business executive Stuart Rose, leader of the new Britain Stronger In Europe campaign, asked.

Prime Minister David Cameron is seeking to renegotiate Britain's relationship with the bloc it joined in 1973 as the 'in' and 'out' campaigns prepare for battle in a referendum on membership that is due to be held before the end of 2017. Opinion polls suggest voters are split, and that crises in the EU over Greek debt and a surge of migrants may be turning some Britons against staying in the 28-nation bloc.

Launching the pro-EU campaign in a fashionable former brewery in East London's Brick Lane, Rose, former chairman of retailer Marks & Spencer, said he wanted reform of Europe but did not want to risk leaving.

"Leaving Europe is a leap in the dark and I don't believe that is a risk that is worth us taking," Rose said. In a speech pitched towards stressing the economic and political pragmatism of staying inside the Union, Rose said the referendum would define Britain's future prosperity.

Both 'in' and 'out' campaigns are well-financed, attracting multi-millionaire backers, and their opposing arguments are keenly honed with many of their proponents having spent years airing them both inside and outside parliament. A rival campaign, Vote Leave, was launched last week by a group of business leaders and lawmakers.

Rose said he supported Cameron's efforts to renegotiate Britain's relationship with the EU, but the country could only do so from within the bloc. "I am not an uncritical fan of the European Union, far from it," said Rose, a Conservative peer.

"Wanting reform is completely different in my view than wanting to leave. Make no mistake I am part of this campaign because I believe that we are stronger, we are better off and we are safer inside Europe than we would be if we were on our own," he said.

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 ensuring the euro is not the EU's official currency, protecting the pound. Rose said the economic argument for the world's fifth largest economy staying in the EU was clear. He quoted figures showing membership was worth £3,000 ($4,600, €4,050) per year to the average British household.

"We know our economy would take a hit, what we don't know is how bad that hit would be," said Rose, adding: "So why would we risk all of that? Surely there is another way; surely there is a better way. Yes, let's prove that we can reform Europe, but let's prove that we can reform Europe by leading within it. By being in Europe, which I believe makes Britain stronger, and that is why I have agreed to chair this campaign to make Britain stronger in Europe," Rose said.