A Brexit could prove to be a living nightmare for unionists as splitting from the EU increases the chances of the UK breaking up, according to Will Straw. The Britain Stronger in Europe executive director issued the warning after IBTimes UK pressed the former Labour parliamentary candidate and Leave.EU co-founder Arron Banks on the issue.

"The polls at the moment suggest that Scottish voters support continuing EU membership at a rate of about two to one, but, overall if the UK voted to leave, that could be a pretext for second independence referendum and quite possibly the break-up of the UK. Let's be really careful and think about the implications of leaving the EU," Straw said.

The comments come after First Minister of Scotland Nicola Sturgeon warned David Cameron that a vote to leave the EU at the historic ballot, which will be held before the end of 2017, could trigger another Scottish independence referendum. "If you try to take Scotland out of the EU against our democratic wishes, you will be breaching the terms of last year's vote," the SNP firebrand declared at an October 2015 conference.

But Banks, who battled through a stomach bug at the Foreign Press Association debate, was confident the union would remain intact after a Brexit. "There's no chance of Scotland leaving, there's no chance of Wales leaving. We've been in the union for 300 years, they had their opportunity, they were at the peak of Scottish nationalism and they still didn't do it," he said.

The two campaigners also verbally jousted over what the implications of a Brexit would mean for UK trade and what role immigration played in the debate. The clash become heated after Straw accused Banks of being an advocate of "dog whistle" politics. Elsewhere, Banks hit out at the rival Brexit group Vote Leave by dubbing it an "SW1 campaign".

The debate came after Britain Stronger in Europe launched its latest attack video. The film mocks top Eurosceptics, such as Douglas Carswell, Nigel Farage and Matthew Elliott, for voicing contradictory statements about what a Brexit would look like. "No one knows what Britain out of Europe would look like – not even those who want us to leave," the video claimed.

The latest opinion poll from YouGov for The Times, of more than 1,500 people between 17 and 18 December 2015, put "leave" one point ahead of "remain" (42% versus 41%, respectively).