Cabinet ministers Kenneth Clarke (Conservative Party) and Danny Alexander (Liberal Democrats) along with former minister Peter Mandelson (Labour) launched the group, called British Influence, on Wednesday (30 January).
The group said in a statement, quoted by AFP, that it was "assembling an army of supporters to fight Europhobia and promote a reform and growth agenda that serves the British national interest".
Cameron promised on 23 January to offer Britons a simple ‘in/out’ referendum choice on whether to stay in the European Union if he wins the next election, scheduled for 2015.
The Daily Mail called it “a provocative move”.
According to advanced copies of the speeches, Mandelson was due to tell the launch event that groups trying to "destroy" Britain's influence in Europe "have been allowed to get away with murder".
Alexander, a junior treasury minister from the Liberal Democrat party which is in coalition with the Conservatives, was to say that Cameron's plan to repatriate significant powers from the EU was "nonsensical."
"We can offer a more compelling alternative: a strong UK, influential in Europe and so more influential in the world. Stronger with America and China, because we are at the heart of the EU," he was to say.
Clark was due to say that it was "obvious" and "essential" that the EU needs to reform in order for the UK to "compete properly in a fiercely global marketplace".
“The real challenge for us as we debate our role in the EU is working out what we should be pushing forward, what we should be leading on within the EU, to make it - and us - a more effective economic and global power,” The Daily Mail quoted from the text of his prepared speech.