A vote to leave the European Union would risk tearing the UK apart, Nick Clegg will warn today in a return to frontline politics.
The former deputy prime minister will use a speech at the Liberal Democrat conference to warn that a vote in favour of Brexit would trigger a fresh referendum on Scottish independence which could lead to the break-up of the union.
He will also urge Labour to wholeheartedly campaign for the UK to remain in the EU after the confusion of Jeremy Corbyn's first few days in power.
Mr Clegg, who quit as party leader following the electoral mauling in May which saw the Lib Dems lose all but eight of their seats, will return to the conference stage in Bournemouth.
His intervention comes as Tim Farron prepares to launch the Lib Dem campaign to remain in the EU ahead of the vote promised by David Cameron by the end of 2017.
Mr Clegg will say: "The stakes could not be higher: not just one, but two, unions now hang in the balance.
"If we vote to leave the EU, I have no doubt that the SNP will gleefully grab the opportunity to persuade the people of Scotland to leave the UK as well."
That would leave a "once great country now pulled apart", creating a "Little England" left isolated and lacking international influence.
"I have no doubt that David Cameron's referendum will be contested on the issue of jobs, economic security, the terms of any renegotiation and so on," Mr Clegg will say.
"But there's a big, enduring question which hangs over all of this: what kind of country do we want to be, what is our role, in this globalised world of ours? Open or closed?
"Leading in our own European backyard or isolated from our nearest neighbours?
"Because let's be clear: for all the huffing and puffing we're going to hear from those who want to leave the EU, they have no answer to that fundamental strategic question."
Mr Corbyn has committed Labour to campaign to stay in the EU after initially giving mixed messages about his response to the referendum.
Mr Clegg will say: "On a number of occasions in recent years I've seen the Labour Party abandon its progressive principles to score short term tactical points: failing to support House of Lords reform; barely lifting a finger in the AV referendum; blocking party funding reform.
"But I say to Jeremy Corbyn: the EU referendum is simply too important for ambivalence."
Mr Farron, who will formally launch his party's referendum campaign later today, said: "Liberal Democrats believe in an open, modern and inclusive Britain that stands tall in the world, and doesn't hide from it.
"We are a modern, free, prosperous country, respected the world over. This is Britain's time to lead.
"By voting to remain in the EU we can secure Britain's position as a world power and build on our role as a beacon of hope, freedom, tolerance and prosperity.
"It is not the time to turn our backs on Europe, and leave us isolated, sidelined and alone."
In other business on the conference agenda, the Lib Dems will vote on a policy which could put ice cream vans out of business - despite Mr Farron's insistence that "Mr Whippy is safe in my hands".
A motion to protect air quality would ban diesel vehicles from leaving their engines running while parked, meaning ice cream vans could face problems running the freezers and machines needed to serve cornets and 99s.
Mr Farron will also face a test of his authority after urging activists to reject a motion which would scrap the Trident renewal programme and lead to the decommissioning of the UK's existing nuclear warheads.