"Britain Stronger in Europe" is being led by businessman Stuart Rose, who highlighted the economic and diplomatic benefits of Britain remaining in the 28-member bloc and warned of the dangers of leaving.
"Do we choose to be stronger in an economy that creates opportunity into the future and the power to shape events?" he said in a speech.
"Or do we choose to be weaker by taking a leap into the unknown, less able to influence global developments that may risk our economy and may compromise our safety?"
The vote could be held as early as next year and could profoundly affect British politics and the country's place as one of Europe's economic hubs.
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Britain's membership of the EU has proven a divisive issue in the past, and has often led to damaging rows within the ruling Conservative Party.
The "In" campaign will include three former prime ministers, former Labour minister Peter Mandelson, and businesswoman and Conservative House of Lords member Karren Brady among other prominent figures.
"Vote Leave", including members of both the Conservative party and the main opposition Labour party, launched last week, while "Leave.EU" - led by the UK Independence Party (UKIP) - was unveiled in September.
UKIP leader Nigel Farage has played down reports of rifts within the camp advocating a so-called "Brexit", saying the two campaigns are "complementary".
He also dismissed the benefits of EU membership laid out by Rose.
"He argued that we are stronger in the world, have more influence and more clout. Has he forgotten that the United Kingdom no longer even has a seat on the World Trade Organisation and that Brussels now wants a single EU seat on the UN Security Council?"
Prime Minister David Cameron, who leads the Conservative government, in 2013 promised to hold the vote amid pressure from UKIP and the eurosceptic wing of his own party.