Boris Johnson has backed Tory backbench demands for an EU referendum bill and warned David Cameron he must make clear Britain is "ready to walk away" unless its relationship is fundamentally reformed.
But the Conservative Mayor of London also issued a stark warning to eurosceptics that quitting would expose the fact that most of the country's problems were self-inflicted.
"If we left the EU, we would end this sterile debate, and we would have to recognise that most of our problems are not caused by 'Bwussels', but by chronic British short-termism, inadequate management, sloth, low skills, a culture of easy gratification and underinvestment in both human and physical capital and infrastructure," he wrote in his Daily Telegraph column.
"Why are we still, person for person, so much less productive than the Germans? That is now a question more than a century old, and the answer is nothing to do with the EU. In or out of the EU, we must have a clear vision of how we are going to be competitive in a global economy."
Mr Johnson's intervention in the fraught Conservative debate over the UK's future in Europe came after two senior cabinet ministers publicly declared that they would vote "no" if the present relationship was put to a vote. He pledged his "full support" to the Prime Minister's strategy of seeking a new pared-down deal to put to the country in an in/out referendum by 2017 if the Tories win the next general election. But he said he wanted to see "legislation now to make sure that referendum goes ahead".
Up to 100 Tory MPs are tipped later this week to back a Commons amendment tabled by backbench eurosceptics expressing "regret" that Mr Cameron failed to include a Bill enshrining the vote pledge in law in the Queen's Speech.
Downing Street has told ministers to abstain in the vote but is allowing other MPs, including parliamentary aides, a free vote, saying Mr Cameron was "relaxed" about them formally criticising his legislative package.
Defence Secretary Philip Hammond earlier joined Education Secretary Michael Gove in saying he would back a British exit if there was an immediate referendum but both backed the Prime Minister's strategy. Mr Gove said he was happy to contemplate cutting formal ties with the EU, suggesting he was in line with public opinion in feeling it would be "perfectly tolerable" and indeed bring some benefits.
Mr Hammond said: "I believe that we have to negotiate a better solution that works better for Britain if we are going to stay in."
Pressure for action from the Tory right has been fuelled by the growing electoral success and influence of the anti-EU UK Independence Party.and calls from several party grandees including ex-chancellor Lord Lawson for a UK exit. Mr Hammond criticised their position as "defeatist" and said he believed Mr Cameron had a "reasonable chance" of securing sufficient reforms.