Nicola Sturgeon’s blueprint for a separate Scotland faltered within hours of her referendum announcement after it was confirmed it would start life outside the European Union and Nato.
The European Commission said the so-called 'Barroso doctrine' continued to apply, meaning a separate Scotland would have to make a fresh application under Article 49 of the Lisbon Treaty, a process that can take many years.
Jose Manuel Barroso, the former commission president, set out the legal view that if one part of an existing EU country became an independent state it would have to apply for membership.
Jean Claude Juncker, his successor, has previously made clear there would be no more states admitted until 2020 – the year after the UK is expected to leave the EU.
Separately, Nato said that a new independent state would have to apply for membership and the 28 existing “allies” would have to consider it.
Although the SNP reversed its long-standing opposition to Nato in the run-up to the 2014 referendum, any Scottish application may be damaged by the party’s opposition to allowing Britain’s nuclear deterrent to stay on the Clyde.
It is thought that Nicola Sturgeon would be unable to start negotiations until Scotland was a sovereign state, a process that could take several years after a vote for separation.
The interventions by the commission and Nato appeared to further undermine Ms Sturgeon’s rationale that the economic “chaos” unleashed by Brexit justified another referendum.
Scotland’s exports to the rest of the UK are worth four time as much as those to the EU and experts have warned of a hard economic border with England if one country is in the single market and the other outside.
But independence movements in Wales and Northern Ireland yesterday sought to capitalise on the First Minister’s demand for a referendum by urging the UK Government to consider their own demands for separation.
Leanne Wood, the Plaid Cymru leader, claimed a vote on Scottish independence would “lead to the end of the UK as a state” and said “in that situation Wales would need to decide its own future.” Sinn Fein demanded an Irish border poll that could lead to the reunification of Ireland.
David Mundell, the Scottish Secretary, said: “I don’t accept this is about Brexit at all. I think Nicola Sturgeon’s irresponsible actions come from her tunnel vision obsession with independence.”
Speaking at a briefing in Brussels, Margaritis Schinas, a European Commission spokesman, said: “The commission does not comment on issues that pertain to the internal legal and constitutional order of our member states.”
But he added: “The Barroso doctrine, would that apply? Yes that would apply, obviously.”
Ms Sturgeon is understood to be considering changing SNP policy so that Scotland would apply in the short term to the European Free Trade Association, the members of which also have access to the single market.
Jens Stoltenberg, the Nato Secretary General, told Sky News that the alliance had no view on the question of independence but warned the rules were clear.
Mr Stoltenberg said: “If it happens then the UK will continue as a member of Nato but a new independent state has to apply for membership and then it is up to 28 allies whether we will have a new member.
“All decisions are taken by consensus, we need the consensus of all allies to invite a new member.” He added: “By leaving the UK it would also leave Nato - of course it is absolutely possible to apply for membership and then the allies will then decide.”