Nick Clegg has warned that leaving the European Union would be "economic suicide" and urged supporters to declare why they want to stay a member of the political bloc.
The Liberal Democrat leader said a "coalition for the national interest" was needed to save the UK before it "stumbled out of the EU".
Clegg accused David Cameron of pledging to hold a referendum in 2017 as a "policy fix" to placate Tory eurosceptics, who themselves had been indulging in "ludicrous myth-making".
Speaking at the London headquarters of Swiss tech firm Buhler Group, Clegg said that Cameron was "playing with fire" with the result that "Britain will get burned".
Brexiters
The Deputy Prime Minister said: "My great fear, in all of this, is that pro-Europeans are being too slow to wake up to the danger ahead.
"The day I dread - the day I hope never comes - is a time when it is all too late: Britain has stumbled out of the EU, and we look back to these days and say we should have done more.
"It will not be enough to speak up on the eve of a referendum. We need to start challenging some of the ludicrous myth-making by the isolationists now."
Clegg's intervention comes after Tory MP Adam Afriyie launched a rebel bid on Sunday to get a referendum on EU membership enshrined in law for 2014.
The Lib Dem leader admitted that a referendum would happen "at some point or another", even if the 2017 pledge was driven by "internal party management".