Leaving the EU could tear the UK apart and create an isolated "little England", former deputy prime minister Nick Clegg will warn today.
Returning to frontline politics for the first time since the Liberal Democrat's disastrous general election results, Mr Clegg will tell the party's conference a vote in favour of Brexit would trigger a fresh Scottish independence referendum.
The former party leader's intervention comes as his successor 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, who quit as party leader following May's election, will tell the conference in Bournemouth: "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."
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 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 the issue is "simply too important for ambivalence".