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* Conservative and Labour heavyweights back pro-EU group
* Campaign vows "political fightback" against eurosceptics
* PM Cameron offers referendum on Britain's EU membership
By Peter Griffiths
LONDON, Jan 30 (Reuters) - Political rivals joined forces on Wednesday to launch a campaign to keep Britain inside the European Union, aiming to turn back a rising tide of animosity towards Brussels that threatens to end its 40-year membership of the bloc.
Politicians of different stripes decided to pool their efforts after Prime Minister David Cameron last week promised to renegotiate Britain's EU membership terms and give voters a chance to say whether they want to leave.
The Centre for British Influence Through Europe (CBIE), a cross-party group that lobbies to keep Britain inside the EU, said it hoped to convince voters that its stance was the "common sense" option in the barrage of rhetoric on Europe.
And two of Cameron's senior ministers urged a "political fightback" against growing calls for Britain to claw back powers from its biggest trading partner or leave it altogether.
Ken Clarke, a Conservative cabinet minister, and Treasury Minister Danny Alexander, of the pro-EU Liberal Democrats, the junior coalition partner, will speak at the group's launch later on Wednesday. They will be joined by Peter Mandelson, a former Labour minister and ex-EU trade commissioner.
"It is fundamentally defeatist to contend that Britain is forever isolated and without a vision in Europe," the group's director Peter Wilding said in a statement.
Britain's long debate pits pro-EU forces who see Europe as a crucial trade partner and source of stability after World War Two against eurosceptics angry with what they see as a meddling and wasteful Brussels bureaucracy that threatens UK sovereignty.
Cameron said last week that his party would campaign for the 2015 parliamentary election on a pledge to renegotiate the terms of Britain's EU membership. He said an "in our out" referendum on the country's membership of the bloc would then be held by the end of 2017 - provided he wins a second term.
While both sides have begun making their case on Europe, a referendum is still far from certain to take place. Cameron must come from behind in opinion polls to win the election, decide which powers he wants to reclaim from Brussels and then overcome European opposition to negotiate their return.
German Foreign Minister Guido Westerwelle underlined the scale of Cameron's task of securing opt-outs from EU rules in an article on Wednesday in which he told Britain: "There can be no cherry-picking."
"We might be conjuring up forces that we can't control," he wrote in the Times newspaper. "We must not put at risk the common ground that we have achieved in more than half a century of European co-operation."
The CBIE will stress the importance of closer ties with Berlin, Brussels and Paris to give Britain a louder voice in the world and to help its trading position.
The campaign was due to be launched at Europe House, an office building which is the London base of the European Commission as well as the UK Independence Party, the anti-EU group which is siphoning off voters from Cameron's Conservatives.
A stone's throw from parliament, it was for years the office of the Conservatives - a party whose divisions over Europe led to Margaret Thatcher's downfall.