Scotland may have to surrender some of its autonomy if it votes Yes to independence but is likely to receive limited additional powers if it votes No, according to economists.

The race to the referendum is "neck and neck" but Scotland faces some "tough choices", Capital Economics (CE) said.

The SNP has promised unconstrained decision-making, tax cuts and increased spending but it may initially have to align with the remainder of the UK on areas such as fiscal policy and military administration, it said.

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But a narrow No vote would not put the demand for more powers to rest, leading to calls for more devolution to Scotland and possibly to London as well, it added.

Alex Salmond has accused the UK Government of "bluster and threats" by refusing to use sterling, but Capital Economics suggests this could be "a better description of the Scottish Government's own position".

"There has to be a possibility that following a Yes vote the Scottish government would eschew triumphalism and offer the UK government a deal," a Capital Economics report on independence said.

"It would offer complete consistency of fiscal policy and banking supervision between the two nations, including, if necessary, tax rises and spending cuts, and hence workable common use of sterling. It would also agree to pay a full part of the cost of debt servicing in return for getting most of the North Sea tax revenues.

"It would accept joint use of all the administrative (and, crucially, military) arms of government that are currently shared, and again would pay its share of the costs."

Both parties would work towards "a gradual and amicable divorce" with staggered deadlines, it said.

An accompanying CE report on devolution said: "It is clear that the opinion polls have narrowed. The likely outcome is neck-and-neck. A narrow No vote would not put the issue to rest.

"Scottish First Minister Alex Salmond has said he would continue to campaign for independence. Meanwhile, all three main Westminster parties would surely offer Scotland 'devo-max', with limited increases in tax and spending powers, as part of their campaigns for the 2015 and 2016 UK and Scottish elections.

"A by-product of that might well be increased agitation for local government elsewhere to receive similar treatment.

"That would be especially so in London: a key battleground in the 2015 general election, and facing a 2016 mayoral election that is also likely to be tight."

Any increase in autonomy for London, which is a quarter of the UK economy, could have "nationwide implications - positive or negative", it said.

Extra taxpayer-funded infrastructure spending "would probably boost, not reduce, competitiveness", but too many taxes, technological advances and the shift to a highly-regulated financial sector may make London firms "less special", it added.

A Yes Scotland spokesman said: "A No means Scotland's future will be in Westminster's hands. The evidence shows there are real costs for people in Scotland when Westminster governments - often governments we didn't vote for - take decisions about our future.

"The jam-tomorrow promises of the Tories and their Westminster partners in the No camp will not be delivered, we believe.

"Nobody, whether they favour a Yes vote or a No vote, should be in any doubt that if we reject independence on September 18 we will pay a very heavy price - more Tory governments that we don't vote for, more austerity and more attacks on those who can least afford it while the super-rich are rewarded with tax cuts."

A Better Together spokesman said the reports highlight risks Scotland "simply don't have to take".

"A currency union between a separate Scotland and the continuing UK would not be good for Scotland or the continuing UK. That is why it is off the table and would not happen. What we need to hear from the nationalists is what would replace the pound."

He added: "Only a vote to remain in the UK will ensure we can continue the success of Scottish devolution. Each of the three Scottish parties backing a No vote are committed to further devolution after September, which will strengthen the Scottish Parliament without losing the back-up that comes from being part of something bigger."

A Treasury spokesman said: "This report from Capital Economics shows the uncomfortable reality for the Scottish Government. Independence means tough choices, lower spending and higher taxes, not vice versa.

"The report also shows that the Scottish Government's threats on matters like the national debt are totally empty and it would not be in a position to assert its terms on matters like a currency union during any negotiations with the rest of the UK. As Capital Economics say, the Scottish Government would be seeking terms with the continuing UK, the EU and others."