Leader of the Labour party’s pro-EU campaign tells Guardian Live debate at the London Palladium that Britain’s unity depends on referendum vote

Scots could be justified in wanting to leave the UK in the event of Britain voting to leave the EU if there was a heavy vote in Scotland against such a decision, according to Alan Johnson, the leader of the Labour party’s pro-EU campaign.

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He made the remark at a lively EU referendum debate organised by the Guardian in London, where the issues of security and immigration loomed large and there were particularly bitter exchanges between the former Liberal Democrat leader Nick Clegg and Ukip leader Nigel Farage.

In a resumption of personal hostilities dating back to the general election, the Ukip leader repeatedly accused the former deputy prime minister of telling lies, at one stage telling a packed audience at the London Palladium that Clegg had made a career out of telling lies for a living.

Johnson warned early on that the UK was running “a very, very high risk not just for Britain’s place in Europe but Britain’s continued unity”.

Admitting that he could understand Scottish people’s desire to leave the UK in the event of a so-called Brexit, he said: “To be frank, if I was a Scot … and yes it does depend on Scotland voting heavily to remain … I would be thinking again as well.”

Representing the leave campaign, Farage and Tory MP Andrea Leadsom rejected the question of a threat to the UK’s unity, with Leadsom insisting that the UK government would not allow the SNP to have a new referendum and Farage claiming that levels of support for a Brexit were on the rise in Scotland.

The debate, which featured noisy interventions from large sections of the crowd, sparked a particularly polarised reaction to the Ukip leader, who drew pantomime jeers from some when he predicted that the EU was on the way to going into a union with Turkey “composed of 77 million even poorer people” and described Barack Obama as “the most anti-British US president”.

He said the biggest threat to the Anglo-American relationship, Britain’s security and Nato came from an EU that was “hellbent on building a European army, navy and air forces as quickly as possibly.”

By attempting to extend its influence, he said the EU had “directly provoked” a coup that got rid of a directly elected government in Ukraine when Viktor Yanukovych was overthrown. “We have actually provoked this situation with Putin,” he said of the conflict in Ukraine.

Representing the remain side alongside Johnson, Clegg denied there was any imminent entry by Turkey, but added: “I think that one of the unusual successes of the EU is that it has encompassed some very different cultures. We have not become any less British since 1970. It isn’t a straitjacket.”

He and Johnson went on the offensive against Farage on the question of immigration. Clegg said Ukip warnings were based on a “a totally false assumption that if we leave the EU people will not continue to try to flee war, attrition and hunger and come to the UK”. He added: “It is so dishonest to somehow claim that if we leave the EU, that problem will go away. It is deeply, deeply dishonest.”

The debate, organised through Guardian Members, was originally scheduled to take place at a much smaller venue in Westminster, but demand for tickets led to the venue being switched to the Palladium, which seats more than 2,000 people.