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 Center 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 on Europe.
And two of Cameron's senior ministers urged a "political fight back" against calls for Britain to take back powers from its biggest trading partner or leave altogether.
Ken Clarke, a Conservative cabinet minister, and Treasury Minister Danny Alexander, of the pro-EU Liberal Democrats, the junior coalition party, were at the launch with Peter Mandelson, a former Labor minister and ex-EU trade commissioner.
Clarke, a Conservative veteran who has served as finance minister and still sits in cabinet as a minister without portfolio, said it would be a "fatal mistake" for Britain to leave.
"That this is not more deeply understood in this country is perhaps because the case has not been made properly for British membership of the EU for many years," he said. "The time has now come for us to put the case more strongly and more coherently."
Britain's long debate pits pro-EU forces that see Europe as a crucial trade partner and source of stability after World War II against Eurosceptic angry with what they see as a meddling and wasteful Brussels bureaucracy that threatens U.K. 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 or out" referendum on the country's membership of the bloc would then be held by the end of 2017 - if 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 and overcome European opposition to negotiate their return.
Alexander, number two to Finance Minister George Osborne at the Treasury, said it was "nonsensical" to suggest Britain should opt out of the bulk of its EU obligations. Mandelson said the anti-EU side had "got away with murder in the lies and false propaganda they have poured out".
The CBIE stresses 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.
Its campaign was 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, the building was for years the Conservative headquarters and the stronghold of Margaret Thatcher - a Conservative prime minister brought down by party in-fighting over Europe.