A LEADING constitutional expert has criticised the Coalition's proposals for a "sudden-death" referendum and warned Scots could be forced to vote without knowing what an independent Scotland would look like.

Professor Robert Hazell said the public could end up buying a "pig in a poke" if Westminster offers the Scottish Parliament a legally binding poll, to be held within 18 months, on the yes-or-no question of whether Scotland should remain part of the UK.

Scots would be asked to vote before key decisions on the make-up of a separate Scotland had been decided, he said.

These include what percentage of the UK's national debt Scotland would be expected to shoulder and the extent of its rights over North Sea oil revenues.

Prof Hazell also said he preferred the Scottish Government's more "gradual" approach, which would have seen a consultative referendum potentially give Scottish ministers a mandate to start negotiations with the UK Government.

Instead, the Scottish public had been offered a "sudden death" referendum, he said.

Key questions about the shape of a future independent Scotland could play a decisive role in any vote, surveys suggest.

Last month an opinion poll found almost two-thirds of Scots would vote to leave the UK if they thought it would make them £500 a year better off.

Any suggestion Scotland could lose out on potentially lucrative North Sea oil revenues could also have a enormous effect on any poll.

Prof Hazell has researched the potential steps that could be required for Scottish independence as head of the Constitution Unit at University College London,

He warned Scots deserved to know what they were actually voting for in any referendum.

"Negotiation would have to include some very big things, including on the percentage of national debt, share of North Sea oil, what to do about the nuclear submarines on the Clyde," he said.

"There is Alex Salmond's wish to share bases [with the UK] for the Armed Forces, how would that work?

"And what about Scotland's membership of the European Union?"

He warned the issues involved in Scotland leaving the UK could potentially be "even bigger" than those which decided the separation of Czechoslovakia, a protracted process which eventually required more than 12,000 legal agreements.

Prof Hazell added: "My worry about the 'sudden-death' referendum is few if any of these issues will have been bottomed out.

"The people in Scotland are being asked to buy a pig in a poke. They are being asked to make a decisive decision on the basis of what is clearly going to be pretty slender information."

Under a "consultative" referendum, he said, of the kind the Scottish Parliament had been expected to hold before David Cameron's dramatic intervention in the debate at the weekend, it could have been expected any final settlement would have to be ratified by Holyrood.

He added that his hunch was there would be a "groundswell" of demands for the public to also have their say, through a second referendum.

However, he suggested both these options would be effectively ruled out by a one-stage referendum process. He added: "I think the gradual approach seems to have a lot of merit. It is orderly, phased and allows thinking and debating time, and could come to a better considered decision than the sudden death strategy."