Pro-Europeans must stop turning a "blind eye" to the failings of the EU and make the case for reform or the country will "sleepwalk" into an economy-wrecking exit, Ed Miliband will tell business leaders.
The Labour leader will tell the CBI conference that pressure on David Cameron from eurosceptic Tories had forced the Prime Minister into "negotiations that will not deliver" for the repatriation of swathes of powers.
Instead he should be concentrating on "building alliances" to agree reforms and ensure Britain did not lose out when eurozone countries deepened their ties in a new two-tier Europe, he will suggest.
Weekend opinion polls showed a clear public majority for cutting ties with Brussels as pressure mounted on the Prime Minister to set out plans for a referendum on the issue. He faces a tricky summit on Thursday when leaders gather to thrash out the EU's future budget, and he has threatened to veto anything other than a real-terms freeze.
Labour joined forces with Tory rebels to inflict a Commons defeat on the Government by backing a real-terms cut in the 2014-20 package - leading to accusations of opportunism. But in a bid to win over the business vote, Mr Miliband will say it is right to press for reform in some areas while remaining "passionate" that membership of the Union is firmly in the national interest.
Quitting the EU would leave Britain "competing on low-wages and low-skills, an off-shore low-value economy, a race to the bottom", he will say, pushing the economic, political and strategic case for membership.
Crises in the eurozone, soaring unemployment, and a budget more suited to the 1950s than the 21st century had however shaken confidence in the Union. New alliances were required to push plans for growth and jobs, re-target spending towards infrastructure, energy and innovation, complete the single market and change state aid rules, he will suggest.
But a rising tide of public scepticism and calls to consider a future outside the EU - including from within the cabinet - had led allies in Europe to believe the "Britain is heading to the exit door". "Those of us, like me, who passionately believe that Britain is stronger in the European Union cannot be silent in a situation like this. I will not allow our country to sleepwalk toward exit because it would be a betrayal of our national interest.
"Reforming the European Union will be difficult, it will require building alliances, and it will have its frustrations. But I am certain it is better than leaving: for business, for jobs, for wealth creation. Our future lies within a reformed European Union that will help us build One Nation."
Senior Tory MP David Davis on Sunday called 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 once they had been negotiated. Mr Davis, a former Europe minister and leadership rival to Mr Cameron, said the first vote should be held within a year, before the next elections to the European Parliament in 2014.