Pro-European Conservatives have begun a fightback against the Eurosceptics, warning that a vote for Britain to leave the EU would be a "jump into a void".
Former minister Nick Herbert – who led the campaign to keep Britain out of the euro 15 years ago – has launched the new Conservatives for Reform in Europe (CRE) group to make the case for Britain to stay in.
At the same time, Education Secretary Nicky Morgan has entered the fray with a warning that young people could find themselves "cut off from the world" if the forthcoming referendum resulted in a vote to leave the EU.
The latest interventions come just days after Leader of the Commons Chris Grayling – an avowed Eurosceptic – said it would be "disastrous" if Britain was to remain in with its current terms of membership.
Prime Minister David Cameron has said ministers will be free to campaign on either side in the referendum – but only after he has completed his re-negotiation of British membership terms which is expected to come to a head at a Brussels summit in February.
He has also warned that ministers must treat each other with "appropriate respect and courtesy" but the increasingly heated exchanges suggest he may struggle to keep the peace among senior colleagues.
They came as a poll for The Mail on Sunday by Survation put support for leaving at 53% against 47% who wanted to stay – although with the polls continuing to fluctuate, both sides will be wary of reading too much into one survey.
Writing in The Sunday Telegraph, Mr Herbert –who is said to have set up CRE with the full support of Mr Cameron - warned that leaving the EU would put investment at risk, undermine policing and security, and jeopardise access to European markets.
"Leaving without the first idea of what we might get instead would be to jump into a void. What matters most to the British people is their jobs and security, living standards and public services," he wrote.
"Whatever our views about the EU, the key long-term challenges facing this this country – how to deliver health and social care with an ageing population, how to increase our competitiveness and productivity, how to deal with our debt and live within our means – would not suddenly be solved by leaving."
In an article for The Observer, Ms Morgan argued that opportunities for young people could be dramatically curtailed if Britain was to leave.
"I think all of us agree what we don't want Britain to be: anti-competitive with more laws made overseas and with people travelling here for the benefits on offer rather than to pay their way," she said.
"But we also don't want our children to inherit a Britain cut off from the world, where their prospects are limited and their opportunities end at our shores."