David Cameron is facing a challenge to hold his party together as battle lines are drawn over Europe.
With just over a week until the Prime Minister's key speech on Britain's relationship with the EU, Tory Europhiles have launched a fight-back against demands for an in-out referendum.
Cabinet minister Ken Clarke will share a platform with Labour peer Lord Mandelson later this month to stress the benefits of remaining in the union.
The move comes after fellow Conservative Lord Heseltine warned that the economy would suffer if Mr Cameron took a "punt" and committed to a national poll on membership.
Around 20 Tory MPs have also apparently signed a letter, due to be published this week, warning of "massive damage" if the UK leaves the EU.
Danny Alexander, Chief Secretary to the Treasury, told Sky's Murnaghan show that he, Mr Cameron and many other MPs were in agreement on the EU's importance to Britain.
The Lib Dem said the idea of isolating Britain from Europe was "just mad". "That would be completely the wrong thing," he said.
"In the end it is our national interest, our national interest in terms of our economy and jobs and society that has to come first in any approach."
Labour leader Ed Miliband on Sunday criticised Mr Cameron's handling of the situation as "incredibly dangerous", and he ruled out promising a referendum before the future shape of the EU was clear.
He told the BBC's Andrew Marr Show: "I think he is essentially sleepwalking us towards the exit door of the EU.
"The last thing we should do is start saying for some date five, six, seven years hence, let's decide now to have an in-out referendum."
Mr Miliband went on: "As Michael Heseltine said very well ... that means you are having a referendum on a negotiation that has not yet begun, with a timescale that is uncertain and an outcome that is unknown. That is an incredible gamble.
"We know why this is happening. (Mr Cameron) is worried about the threat from UKIP and he is worried about what is happening in his own party."
Rumours have been circulating that Downing Street has given tacit approval to efforts to highlight the dangers of an exit.
In an unusual intervention last week, senior US diplomat Philip Gordon openly stated that America wanted Britain to remain in the EU.
Prominent business figures including Sir Richard Branson have also spoken out about the potentially dire consequences of severing ties.
Tory backbencher Robert Buckland, who has organised the pro-membership letter, said he had been informed that Number 10 regarded his efforts as "helpful".
"There is a silent majority out there who do not want Britain to leave the EU," he told the Mail on Sunday.
"The danger for the Tories is that because the right-wing Eurosceptics are making the most noise, we could slide towards the exit door of the EU."
Meanwhile, a ComRes poll has put UKIP above the Conservatives when people were asked about who they would vote for in the European elections - 35% said they would back the Tories, with 23% saying UKIP. The Conservatives came third with 22%, followed by the Lib Dems on 8%.
UKIP leader Nigel Farage told Sky News: "If it is a protest, there's a lot to protest about because we've got three so-called main parties who resolutely do not want to give the British people a say on their future in the EU."