The Scottish Government must back up its assertion that an independent Scotland would remain a member of the European Union (EU), according to the Prime Minister's most senior adviser on Scots law.
The more likely consequence of independence is that Scotland would have to apply to join the EU as a new state, Advocate General for Scotland Jim Wallace insisted.
It is unclear whether the Scottish Government has sought legal advice to back up its EU assertion because ministers have gone to court to prevent the public knowing if it exists.
Lord Wallace concedes that the UK Government takes the same view on the confidentiality of its own legal advice. But in a speech to the Edinburgh Centre for Constitutional Law (ECCL), he will argue that this does not prevent the Scottish Government from setting out its understanding of EU law in more detail.
"I think the time has come in this debate where we need to move away from mere assertion and look in more detail at the issue, having regard to both international and EU law and to international legal precedent," he said. "The Scottish Government is not precluded by any code from setting out in detail its understanding of the law on an important issue such as this."
He questioned whether Scotland's EU "accession" agreement would include the UK's opt-outs from the euro currency and the Schengen passport-free travel agreement.
The House of Commons library has set out three legal possibilities: Scotland and the rest of the UK retain EU membership equally; Scotland is ejected but the rest of the UK is not; or both are ejected and have to reapply.
Lord Wallace argues that it would be "against both precedent and common sense" for the rest of the UK to be ejected from the EU in the event of Scottish independence.
"When the Irish Free State was established in 1922, the United Kingdom continued to exist," he said. "The Russian Federation, for example, may seem a much smaller entity than the Soviet Union, but despite several new states having been formed from parts of the old Soviet Union, the Russian Federation was recognised as the continuation of the Soviet Union. Even the independence of India and Pakistan in 1948 involved the creation of one new state, Pakistan, and a continuing state, India.
"The more likely conclusion, and the one which people must weigh in the balance before casting their vote, is that the UK would carry on with its international obligations including EU membership, as at present. The likely consequence is that Scotland would have to apply to join the EU."