David Cameron pleaded with Tory MPs not to back a referendum on Europe tonight - warning the move would damage Britain's interest.
The Prime Minister attempted to placate his rebellious rank-and-file ahead of a crunch Commons vote by insisting the chance to repatriate powers from Brussels was coming.
But he insisted those issues should not be addressed while the Eurozone was "on fire" with the sovereign debt crisis.
The direct appeal, in a statement to parliament, came amid claims that two ministers and more than half a dozen ministerial aides could defy the Government.
More than 60 backbenchers have also said they will vote for a referendum on membership of the EU, in the most serious challenge to Mr Cameron's authority since he became party leader six years ago.
Conservative MPs have accused the leadership of bullying tactics and bungling party management by imposing a strict three-line Whip on the motion.
There has also been a furious response to efforts by Foreign Secretary William Hague to quell the rebellion, with some backbenchers saying he had "gone native" and abandoned his Eurosceptic values.
Delivering a statement to the Commons, Mr Cameron said it was essential to restore growth in the EU by stabilising the Eurozone and boosting free trade.
But he argued that an "in-out" referendum was the wrong approach at the wrong time.
"It's not right because our national interest is to be in the EU, helping to determine the rules governing the single market - our biggest export market, which consumes more than 50% of our exports and which drives much of the investment into the UK," he said.
"That is not an abstract, theoretical argument; it matters for millions of jobs and millions of families in our country.
"That's why successive prime ministers have advocated our membership of the EU."
He warned that launching legislation for a referendum could be disastrous at the current "moment of economic crisis".
"When your neighbour's house is on fire, your first impulse should be to help him put out the flames - not least to stop the flames reaching your own house," he said.
"This is not the time to argue about walking away. Not just for their sakes, but for ours.
"Legislating now for a referendum, including on whether Britain should leave the EU, could cause great uncertainty and could actually damage our prospects of growth."
Mr Cameron said there was also a danger that a referendum could wreck prospects for securing the change that Eurosceptic Tories crave.
Fundamental reform to EU treaties would require support from all 27 nations - giving the UK an effective veto.
"Opportunities to advance our national interest are clearly becoming apparent," he said.
"We should focus on how to make the most of this, not pursue a parliamentary process for a multiple choice referendum."
The premier made an appeal to moderate Eurosceptics, saying: "I respect your views. We disagree not about ends, but about means.
"I support your aims. Like you, I want to see fundamental reform.
"Like you, I want to re-fashion our membership of the EU so that it better serves this nation's interests.
"The time for reform is coming. That is the prize.
"Let's not be distracted from seizing it."
The premier said he had made the decision to impose a three-line Whip rather than allow a freer vote because "this issue and Parliament matters".
The result is not binding, and Labour and Liberal Democrat support is almost certain to mean the motion will be defeated.
However, a huge revolt and resignations from government could be highly damaging for the Prime Minister. The previous largest Tory rebellion over Europe was in 1993, when 41 MPs defied John Major on the Maastricht Treaty.
Mr Cameron has postponed a foreign trip to vote personally and spent the day meeting potential rebels as he seeks to defuse the row.
In a dramatic episode, one PPS, thought to be Mark Menzies, fainted during one of the sessions with the premier.
He was treated by paramedics and taken to hospital, according to Mr Cameron's spokeswoman.
Stewart Jackson, PPS to Northern Ireland Secretary Owen Paterson, has pledged to vote for a referendum even though it will cost him his job.
Mr Jackson told Sky News the Tories "all wanted to get to the same place, but we're doing it in different ways".
But he said he disagreed with Mr Cameron's appeal for patience. "When is the right time?" he asked. "If not now, when?"
Mr Hague angered rebels further earlier by comparing the Commons vote on a referendum with a piece of "graffiti".
"Clearly our whole relationship with the European Union is a matter that concerns the government as a whole and not just something for the House of Commons to put up some graffiti about," he told BBC Radio 4's Today programme.
Wellingborough MP Peter Bone said the Conservative rank-and-file no longer trusted the Foreign Secretary.
"The trouble with William Hague is that people just don't believe him on the backbenches," he said.
"We think Hague has gone on a travel. He has gone from being Eurosceptic to someone who has got into this ministerial, political world of Europe and loves it."
Wrekin MP Mark Pritchard, secretary of the powerful Tory 1922 committee, said the debate would be "a defining moment for many MPs who have for years called themselves Eurosceptic".
The Commons will stage a single vote on the issue after Speaker John Bercow rejected compromise amendments to the original motion.
The text calls for a Bill to be introduced providing for the holding of a referendum asking whether the UK should stay in the EU on current terms, leave entirely, or renegotiate its relationship.
Labour leader Ed Miliband told MPs Britain "could not afford" to leave the EU at the moment and should concentrate on pushing through reforms.
But he mocked the Tories for having another "nervous breakdown" on Europe.
PA
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