Labour and the Liberal Democrats are turning up the heat on David Cameron as he struggles to stop his party tearing itself apart over Europe.
With just over a week until the Prime Minister's crunch speech on Britain and the EU, Ed Miliband insisted it would be "wrong" to promise a referendum, and accused Mr Cameron of "sleepwalking us towards the exit door".
Meanwhile, Lib Dem Cabinet minister Danny Alexander warned that the consequences of leaving the grouping were so drastic that no responsible politician would contemplate it.
The interventions came as Tory Europhiles launched a fightback against demands for an in-out referendum.
Cabinet minister Ken Clarke is to share a platform with Labour peer Lord Mandelson later this month to stress the benefits of remaining in the union. The pair are launching a new cross-party organisation, the Centre for British Influence through Europe (CBIE), to make the "patriotic" case for British engagement.
Around 20 Tory MP have also apparently signed a letter, due to be published this week, warning of "massive damage" if the UK leaves the EU. Rumours are swirling 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 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".
"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," he told the Mail on Sunday. "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."
Another Conservative 'Big Beast', Lord Heseltine, waded into the row on Saturday by warning that the economy would suffer if Mr Cameron took a "punt" next week and committed to a referendum on membership. However, the party's 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.