Ed Miliband has called for major reforms of the European Union and a "hard-headed" approach taken by Britain, ahead of a crunch EU budget summit this week.
The Labour leader - who last month joined forces with Tory rebels to defeat the Government over its strategy to freeze the EU budget - said Labour must not ignore the legitimate concerns of eurosceptics.
He said reform was needed on the budget, on immigration rules, state aid restrictions and austerity measures - but declined to promise a referendum on the UK's future within the EU.
"What I would say is: never shrink from being open about the problems of the European Union," he told the Sunday Telegraph.
Mr Cameron travels to Brussels on Thursday facing pressure from his backbenchers to push for the real-terms spending cut approved in the non-binding Commons vote Labour helped secure.
The Prime Minister, who insists a rise in line with inflation is a more realistic target in the negotiations, has threatened to use the UK's veto if the rise proposed by the commission is not drastically reduced.
He is under mounting pressure to set out plans for a referendum, with restless MPs increasingly concerned about the threat posed to the Conservatives by the UK Independence Party at the next general election.
Ukip scored its best parliamentary by-election result in Corby, coming in third ahead of the Liberal Democrats and an opinion poll today showed more than a quarter of Tory supporters would "seriously consider" switching.
Mr Miliband - who told French president Francois Hollande in the summer that he saw Britain's place as "firmly in Europe" - was accused of opportunism for voting with the Tory rebels.
But he insisted his party could be at once a keen supporter of EU membership and "realist" campaign for reform.
Mr Miliband, who is due to reinforce his points in a speech to business leaders at a CBI conference on Monday, said he believed bosses were "genuinely worried that we're going to sleepwalk towards an exit under Cameron".
"Nobody thinks he's at those negotiations with anything other than with an arm up his back from the people in his own party," he told the newspaper.
"People are always writing us off as if to say that these guys are going toward the exit. That's very dangerous for us."