History will "never forgive" the Liberal Democrats if they "sit on the fence or - even worse - act as a catalyst for Britain's withdrawal from Europe," Andrew Adonis warns in an interview with the New Statesman.

The remarks came after Nick Clegg gave the clearest indication yet that he would prefer another coalition with the Conservative Party. Clegg told the Financial Times that he would not join a coalition dependent on “life support” from the SNP, and suggested that a government led by a party that finished second – most probably Labour - would lack “legitimacy” in the eyes of the electorate. But Adonis - who prior to joining the Labour party was a member of SDP - remains hopeful that a deal can be struck between the parties.

“We are campaigning for an overall majority and we hope to win one,” he told the New Statesman. “But if there isn’t then I very much hope the Lib Dems will be true to their values and their roots and turn left rather than right.” It is a reminder of Labour’s desperation to refute the Tory charge that they could only govern with the support of the SNP.

Adonis warned Clegg and his party of what another deal with the Conservatives would mean, especially for Britain’s role in Europe. “The history books will never forgive the Lib Dems if they sit on the fence or – even worse – act as a catalyst for Britain’s withdrawal from Europe,” he said. “The first act of that government would be to fire a starting pistol for the European referendum that could have Britain out of Europe within two years. I simply cannot believe that Nick Clegg would wish his legacy to be Britain’s exit from the European Union.”

Adonis suggested that the Conservatives had “moved to the right” in the last five years, making a future deal with the Lib Dems harder. “These are much wider issues than in 2010 when Europe wasn't a dividing line,” he said. “It looks to me to be very hard indeed to see how a principled government of the Conservatives and Lib Dems could be put together.”

“The reality is that whatever David Cameron says, the Conservative membership at large will be campaigning for exit and it’s perfectly possible that a majority of Conservative MPs will as well.”

But Adonis suggested that the obstacles to a Con-Lib deal extended way beyond Europe, highlighting the Tory plan for a further £12 billion in welfare cuts. “The scale of welfare cuts are of a whole order larger than in the last Parliament, which would undermine the Lib Dems claim to being a party of social justice,” he said, also highlighting different Tory and Lib Dem approaches to the NHS and the pace of spending cuts.

“I see the barrier to a Conservative-Lib Dem coalition as being very personal barriers to Nick Clegg as well as being a personal philosophical challenge for his party.”

Still, Adonis admitted that a second Con-Lib deal was possible. “Nick has gone in with the Tories once so its conceivable that he could do so again – but he would do so from a fundamentally weaker position both in terms of the Lib Dems but also in terms of the Conservatives and its very hard to see how this would be sustainable for any length of time even if a Tory-Lib Dem coalition were to keep Britain in Europe.”

Adonis warned his former party that a second Con-Lib coalition would have disastrous consequences for their future. “Across much of the country the Lib Dems have simply vanished as a party of consequence,” he said. “Another five years of coalition with the Conservatives and I doubt they'll be much left of the Lib Dems as an organised political force outside of the few areas where they continue to have MPs.”

Amid the warnings to Clegg’s party, Adonis tried to give a positive vision of what a future Lib-Lab coalition could achieve, suggesting the parties were much closer together than when Labour was in government.

“Clearly before 2010 Labour and the Lib Dems fell apart seriously – Iraq was a running sore and Lib Dems regarded Labour as increasingly illiberal on civil rights issues such as ID cards. Most of these issues have now vanished from politics and our underlying unity can now assert itself much more freely and I hope it will do so.”

“Philosophically there’s very little on which we disagree, however much we may argue about individual policies,” he said. A coalition would display “absolute commitment to keeping Britain within the European Union and following fairer social and economic policies.”

“The pro and anti EU issue is going to continue to continue to divide politics in the decade ahead and it’s important that both Labour and the Lib Dems are unambiguously on the pro-European side of this argument.”

And, while Labour might be downhearted by Clegg’s comments, Adonis did not view them as ruling out a Lib-Labour deal.

“He’s said that the Lib Dems would talk first to the party with the largest mandate,” Adonis said. “I’m assuming that he would speak to both parties as last time - particularly since it appears unlikely that there’d be such a large gap between the parties next time, whoever is first or second.”

“What he hasn’t done is ruled out a government with the second party if that produces a stronger, more principled government.”