* Blueprint to spell out Scotland's case for independence

* Questions remain over currency, EU, economy

* Separatists lagging in opinion polls

By Belinda Goldsmith

GLASGOW, Nov 26 (Reuters) - The Scottish government will focus on the potential economic gains from independence on Tuesday when it unveils its vision for the future if Scots choose to end a 306-year union with England.

Alex Salmond, head of a devolved government in Scotland - which for now is still part of the United Kingdom - has promised to spell out exactly what would happen if Scots vote on Sept. 18 next year to leave the UK.

With separatists lagging in opinion polls, his Scottish National Party is hoping the 670-page blueprint will win over the many sceptics, answering questions the SNP has been accused of dodging, such as the currency of an independent Scotland, its membership of the European Union and border arrangements.

"We are setting out a positive plan for job opportunities and economic growth based on Scotland's vast natural resources, key growth sectors and human talent," said Salmond, whose party has a majority in Scotland's parliament.

Nicola Sturgeon, the deputy first minister, has described the document as "the most comprehensive and detailed blueprint ever drawn up for a prospective independent country".

Scotland's bid for independence is being watched closely internationally, particularly in Catalonia where 80 percent of people favour a vote for independence from Spain.

"If it's feasible in the UK, it should be feasible in Spain," said Albert Royo, secretary general of Diplocat, the Public Diplomacy Council of Catalonia, a public-private body charged with building support for Catalan's independence vote.

WAVERING VOTERS

With 10 months to the Scottish vote, many of the 5 million Scots are still undecided.

The latest poll, published in the Sunday Times this week, suggested the gap had narrowed with 47 percent opposed to quitting the UK, 38 percent in favour and 15 percent undecided.

Britain's three main UK-wide political parties have argued against independence, saying Scotland would be worse off economically on its own and unable to defend itself or project power on the global stage as well as it can as part of the UK.

At stake are British oil reserves in the North Sea while debates over how Britain would split its debt and what it would do with its Scotland-based nuclear weapons - which the SNP vows would have no place after independence - are already fraught.

The pro-unionists have been helped in recent weeks by two reports from financial institutions. One warned Scotland would need to raise taxes and cut spending as North Sea oil revenues decline and its population ages and the second said independence would complicate cross-border pensions.