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 forced 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.
Mr Miliband, who is due to reinforce his points in a speech to business leaders at the CBI conference on Sunday, 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."
Shadow foreign secretary Douglas Alexander told the BBC Andrew Marr programme Labour was "very proud" of Britain's role in Europe and that withdrawal was not an option.
"We are very proud of Britain's role in Europe in recent decades and the fact we have seen peace and prosperity in a continent twice in the 20th century," he said. "But it doesn't help the pro-European case to suggest the status quo does not need change - change is coming to Europe and that is why we will remain a pro-European, pro-reform party, taking a hard-headed view of what Europe does well and what Europe does badly.
"Frankly we see the future as Britain being reforming in Europe, not exiting from Europe, and there is a growing number of Conservatives who believe the latter is the way forward."
He dismissed as "nonsense" a call by senior Tory MP David Davis for a double referendum - one to approve a list of powers for the UK to seek to seize back and then an in/out public poll.
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.