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.”
Former deputy prime minister Nick Clegg said he hopes the “millions of people who don’t really care” very much about Britain’s membership of the EU will vote for the country to keep it.
The pro-EU Liberal Democrat MP told BBC One’s the Andrew Marr Show: “The people who really count in this referendum – namely the people who don’t either passionately believe we should stay or passionately believe we should leave, millions of people who don’t really care about this very much – I suspect at the end of the day, I hope at the end of the day, they will believe that the risks of leaving outweigh the imperfections of staying.
“Of course, there are imperfections in the European Union, much as there are in our political arrangements here.”
Mr Clegg also insisted mass migration will remain a “problem” regardless of whether Britain stays in or leaves the EU.
He said: “The idea that if we were to pull out of the European Union, suddenly the problem, the challenge of people moving large distances away from conflict and destitution from the Middle East, from Syria, from Africa, that that will somehow go, is a nonsense.
“We have an arrangement, for instance, which we entered into with France back in 2003 within the context of our shared membership of the European Union, such that they check passports and we can check people’s entry into the United Kingdom on French soil.
“There’s no reason to believe that would necessarily continue if we were to pull out of the European Union, so I just don’t buy this idea that by turning our back on Europe somehow we can wave a magic wand and imagine that mass migration won’t be a problem.
“It will remain a problem whether we remain in or out. My view is, in an uncertain, insecure, unsafe world, there’s safety in numbers.”