He added that Scotland would be forced to use the euro as its currency if the country is allowed to join the EU after another referendum on independence, which Ms Sturgeon is considering following the UK’s Brexit vote.
The First Minister is poised to launch a “hearts and minds” push to win over undecided voters in the coming weeks after indicating that a second vote on leaving the UK is “highly likely” in the wake of Brexit.
Lord Steel, the former UK leader of the Liberal Democrats, believes a second independence referendum is unlikely to be imminent following claims at the weekend that the SNP is planning to hold one next year.
He said: “I know we voted very firmly – and that’s good – to stay in Europe, but the obstacles in the way of trying to go it alone are enormous.
“For a start Scotland would have to try to get the votes of every single country of the EU to be admitted to the EU, which is not impossible but very difficult given the antipathy in some other countries to any suggestion of splits within their countries.”
The SNP are reviewing their currency policy after the 2014 plan for a currency union with the remainder of the UK was widely criticised.
Lord Steel insisted the euro would be inevitable as part of EU membership.
“They would have to accept the euro as a currency. In a way that would get the SNP out of a hole because the last time they didn’t know what currency they would have, so maybe they would accept the euro.
“That’s the rule – new members of the EU have to join the euro; there’s no ifs and buts about it. Similarly they would have to accept the Schengen Agreement, which is free movement of people.
“I think it’s much more difficult than people realise.”
The SNP has pointed to Sweden, which joined the EU in 1995 with a commitment to join the euro, but has yet to adopt it.
Lord Steel said the First Minister’s recent meeting with Prime Minister Theresa May was a “good one”.
He said: “She’s right to keep her oar in the Brexit process, but I don’t really see an independence referendum on the horizon. I think we will have to wait and see what happens to the Brexit campaign first.”
Scots voted in favour of the UK staying in the EU, with 62 per cent of voters north of the Border wanting to remain in the Brussels bloc, while 38 per cent wanted to leave. This compares with 52 per cent UK-wide who voted Leave.
Three polls conducted in the immediate aftermath of the Brexit vote in Scotland indicated that there was a small majority in favour of Scotland leaving the UK to become an independent country.
A more recent poll by YouGov suggested that support for independence was around the 47 per cent mark, which is higher than it was at the time of the referendum in 2014.
The First Minister has established an expert panel which is looking into possible scenarios which could allow Scotland to protect its relationship with the EU as part of the UK.