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.
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 and PR guru Roland Rudd have also spoken out about the potentially dire consequences of severing ties.
Sources told the Mail on Sunday (MoS) that Mr Cameron believes it is "mad" to think that Britain can go it alone.
And 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 MoS.
"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."
Mr Buckland added: "I have been told No 10 views my efforts as being helpful. The Prime Minister is a Eurorealist. He wants us to stay in the EU while having a debate about the terms of our membership, but it must not be used as a Trojan horse to get us to leave."
According to the Observer, Mr Clarke and Lord Mandelson are spearheading a new organisation, the Centre for British Influence through Europe.
The group, due to launch at the end of the month, will apparently support a cross-party "patriotic fightback for British leadership in Europe".
However, Tory Eurosceptics are determined to maintain pressure on Mr Cameron, buoyed by Chancellor George Osborne's recent comments that the UK can only stay in the EU if it changes.
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