Scottish voters would be justified in pushing for independence if the UK voted to leave the EU, but Scotland voted to stay in the 28-member bloc, Alan Johnson has warned.
Johnson said that the UK was running a "very, very high risk not just for Britain’s place in Europe but Britain’s continued unity”.
"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," he added at a debate hosted by the Guardian.
Johnson's claim was rejected by Conservative MP Andrea Leadsom, who said the UK government would not allow a second referendum, given the country had had one recently.
Her comments echoed that of the Prime Minister, who has previously said the Scottish independence referendum was a "once in a generation" event.
UK Independence Party leader Nigel Farage, also in attendance, questioned polls which have suggested Scottish voters are heavily in favour of remaining in the EU. He said support for Brexit was increasing in Scotland.
First Minister Nicola Sturgeon has already indicated she would be very likely to regard the UK voting to leave the EU, but Scotland voting to remain, as a "trigger" justifying a second referendum.
But Farage also claimed that justification for independence could be questioned given the price of oil plummeting since the referendum back in 2014.