SNP veteran Alex Neil warns voters will reject independence if it means rejoining the EU and erecting "customs barriers" at the border.
The ex Scottish Government minister was the only one in Nicola Sturgeon's cabinet to support Brexit.
Today he said there would a "serious and significant" border with England if it was outside the EU while Scotland was back inside the trading bloc.
Neil told ITV Border: "You've got to be honest about these matters."
His comments will reignite debate within the SNP about the best strategy to argue for independence. However, polls show the party's current message is finding support with a record 55% saying they would back a Yes vote in a second independence referendum.
Sturgeon is firmly in favour of rejoining the EU and argued strongly against Brexit.
Neil, who is standing down at the next Holyrood election in May, said: "If Scotland was in a customs union, ie the EU, and the rest of the UK was out of the customs union, by definition there are going to be customs barriers at the border between Scotland and the rest of the UK, and I don’t believe people in another independence referendum will vote for that.
"There is a better alternative, and that is for an independent Scotland to join the European Free Trade Association.
"That gives us free trade with the entirety of Europe, not just those in the EU, but the entirety of Europe through what's called the European Economic Area with out the downside of being in a customs union, or many other downsides on fishing and other things in relation to EU membership."
