The British Government intensified its campaign to stop Scotland leaving the United Kingdom, publishing a legal opinion saying it would forfeit its membership of international bodies such as the European Union if it chose independence.

The pro-independence Scottish National Party (SNP) that runs Scotland’s devolved government plans to hold a referendum on the emotionally charged subject next year, and has played down the impact of a “Yes” vote on Scotland’s inter­national status.

But the 57-page legal opinion – drafted for the UK Government by two independent experts on international law – said the implications could be far-reaching, likening the situation to the 1991 collapse of the Soviet Union when Russia was dec­lared the USSR’s legal successor but the 14 other Soviet states had to forge their international relations anew.

The overwhelming weight of international precedent suggested Scotland would be legally deemed a “new state”, it said – a scenario that would force it to re-apply to join international bodies such as the EU, the United Nations and Nato.

The Government’s intervention came as a panel of experts, including two Nobel prize-winning economists, issued a report saying the SNP’s plan to keep the British pound in the event of independence was a sound strategy, suggesting it would also be wise to keep the Bank of England as the central bank.

The SNP argues that North Sea oil revenues combined with Scotland’s fishing, farming and whisky industries would be enough to keep an independent Scotland solvent. But critics say the oil is running out, that Scotland would lose disproportionately generous British Government subsidies, and that it would struggle to raise enough tax to pay its bills.

The UK Government’s unusual decision to publish the legal opinion reflects its concern that Scots may vote for independence, triggering the break up of a United Kingdom comprising England, Scotland, Wales and Northern Ireland.

European Commission President José Manuel Barroso also believes Scotland would be “a new state”.

When asked about it, he has repeatedly said that any new state that breaks away from an existing EU state would have to re-apply to join the bloc.

Prime Minister David Cameron intervened in the British debate on Sunday, conceding that Scotland had what it takes to be an independent nation, while arguing it enjoyed “the best of both worlds” as part of the UK.

“Put simply: Britain works. Britain works well. Why break it?” he wrote in an article published in Scottish newspapers.

Cameron’s political future and historic legacy are on the line. He has pledged to contest the next British election in 2015 and his own Conservative party would never forgive him if he presided over the break up of the UK.

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