Scotland would have to wait up to 10 years to re-join the EU if it decides to opt for independence from the UK, according to the Institute for Government (IfG).
The think tank said it would require the best part of a decade for the process to be sorted out, and would “inevitably” lead to the emergence of a hard border with England.
However, a new report from the IfG found that an application to join the bloc would almost certainly mean a long wait and Scotland having to adopt the Euro as its currency.
“The EU would probably welcome an application from an independent Scotland, but only if Scottish independence were based on agreement with the UK government,” the think tank’s report states.
“Under EU law, Scotland could only formally apply to join the EU once it had secured its independence from the UK, and the whole process could take the best part of a decade.”
The academics found that joining the EU would probably mean an independent Scotland joining the single market and customs union – and as a result the Anglo-Scottish border would become a new regulatory frontier for the EU.
Even if a looser model of integration was adopted, such as Scotland joining the European Economic Area (EEA), it would not mean frictionless access to both the EU and the UK markets.
“As an EU member state, Scotland would have no choice but to enforce customs processes, as well as regulatory checks on goods such as animal and plant products,” the IfG report states.
The authors of the London-based think tank, viewed as neither left nor right-wing, added: “There would be a need for new border infrastructure to enforce these rules.”
Noting that Scottish businesses trade roughly three times as much with the rest of the UK as with the EU, the authors said that the SNP needed to be “open” about the “costs as well as benefits” of EU membership.
Ms Sturgeon clashed with rival Scottish political leaders during Tuesday’s BBC Holyrood election debate.
The SNP chief claimed Scotland was in danger of going in the “wrong direction” if it left decisions to Boris Johnson’s government, adding: “It’s up to the people of Scotland to decide.”
However, Scottish Labour leader Anas Sarwar said: “If she obsessed about fighting poverty as much as she does about the constitution, imagine how different Scotland could be as a result.”
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