Anything other than a decisive No vote in the Scottish referendum risks an economic slump in the country, banking giant UBS has warned.

A narrow victory for the No campaign and a retention of the union could still lead to a downturn as uncertain investors may be spooked by the possibility of another referendum in the near future, the bank said.

UBS economist Paul Donovan said the so-called "Quebec scenario", named after the economic dip the province suffered after narrowly rejecting independence from Canada in 1995, could threaten jobs and investment.

Mr Donovan also raised the possibility in the event of a Yes vote that the UK monetary union may not last the transitional phase before independence as savers with Scottish bank sort codes "scramble" to take their money out of the country.

With the polls still tight but currently indicating a narrow No vote on Thursday, Mr Donovan said that First Minister Alex Salmond's assurances that the referendum is a "once in a generation" opportunity may fall on deaf ears in the markets.

Such a situation occurred in Quebec despite assurances from its premier that there would not be another referendum, and its economy is still under performing today, he said.

At a briefing in the City, Mr Donovan said: "Although the premier of Quebec in the aftermath of the referendum was in his own mind very clear that there would be no further referendum, the opinion of the market, and indeed the opinion of the Parti Quebecois was that another referendum was likely within a matter of two or three years.

"That then adds a risk premium into markets because of course what you are creating is period of prolonged uncertainty."

Mr Donovan said money which had been pulled out of Quebec banks did not return and savers stopped putting their money into local financial institutions.

"In other words, if you were opening a new bank account and you were Quebecois you would open it over the border and of course we then saw a decline in the bank deposit to GDP ratio of Quebec which lead to a decline in financial sector employment," he said.

"Investment in Quebec also underperformed very significantly after 1995, the investment to GDP ratio in Quebec prior to 1995 had generally matched that of the rest of Canada.

"After 1995 it underperformed by a matter of 2 to 3% of GDP for over a decade, in fact it is still under performing today.

"I don't think we can attribute all of that to the Quebecois scenario, there are other factors that can go on, economies are complex, that I think was one factor amongst many that contributed to a downturn in investment."

In the event of a Yes vote it is likely that savers would move to take their money out of Scotland and put it into banks in the rest of Britain in a "sort code scramble", Mr Donovan said.

If this happened at a rate that politicians could not keep up with, as when Czechoslovakia broke up in 1992, the monetary union between Britain and Scotland could collapse before independence, he warned.

In any event savers pulling out of Scotland en masse could cause a credit crunch and threaten Scottish jobs.

"Every monetary union break-up without exception in the 20th century was preceded by a sudden deposit transfer from one part of the monetary union to another," Mr Donovan said.

"And I think that there would be in the wake of a Yes vote fairly immediate concern, a scramble on the part of the bank depositors to find out whether there sort code was Scottish or British.

"And people would be looking to have their money domiciled in Britain."

He went on: "The deposit transfer of monetary union breakups in the 20th Century has always happened faster than politicians can react.

"When people savings are concerns the economic theory of loss aversion kicks in, people don't like losing things, they really don't like losing things, and they will take very rapid steps to prevent that.

"The best example of this I think is the Czechs and Slovaks. When the Czechs and Slovaks announced their separation in 1992, the plan was that the monetary union would continue and they guaranteed that the monetary union would continue at least for six months.

"It lasted less than six weeks because people were moving cash electronically and ultimately in suitcases from Slovakia to the Czech Republic and the monetary union could not cope with the strain and collapsed."

But on the brighter side, Mr Donovan said some workers could have a day off on Friday if banks are given a holiday in order to cool off and come up with a response to a breaking up of the union.

"It is worth mentioning that in the face of rapid deposit transfer in the United States when that monetary union broke up in '32-'33, the response was to declare a bank holiday which Roosevelt did in March 1933 in order to allow the banks time to cool off and come up with a response.

"I wouldn't bear to predict it but we may get Friday off, you never know."