Britain must take a "hard-headed" approach to the problems facing the EU, Ed Miliband said as he sought to pile pressure on David Cameron ahead of a crunch EU budget summit next week.
The Labour leader - who last month joined forces with Tory rebels to defeat the Government over its strategy - said Labour must not ignore the legitimate concerns of eurosceptics.
Reform was needed on the budget, on immigration rules, state aid restrictions and austerity measures, he told the Sunday Telegraph - 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 said.
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 one 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 the CBI conference tomorrow, 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 said - leading other European leaders to not take Britain seriously.
"People are always writing us off as if to say that these guys are going toward the exit. That's very dangerous for us."
Meanwhile, an opinion poll found that well over half of British voters would vote to leave the European Union if a referendum was held.
A total of 56% said they wanted the UK to cut ties with Europe, while 30% wanted to remain in the EU, the Opinium survey for the Observer revealed.
In terms of political party, 68% of Tory voters want to leave, compared with 24% who want to remain, while 44% of Labour supporters would vote for the UK to go it alone against 39% who want to stay in Europe.
Around 39% of Liberal Democrats want out, compared with 47% who would vote to remain in the EU.