Scotland's leading banks warned Thursday that they would move their headquarters to England if Scots vote to leave the United Kingdom, intensifying doubts about the territory's economic future and dealing a blow to the Scottish independence campaign just days ahead of a historic referendum.

Although the banks say the contingency plans are legal procedures that would have a minimal effect on their operations and jobs in Scotland, the warnings renewed concerns about an independent Scotland's ability to retain businesses — particularly during the long months of financial uncertainty that would follow a vote to break the 307-year union with England.

The Royal Bank of Scotland, which has been based there since 1727, said it drew up the plans because of uncertainties that could hurt its business and customers if the Sept. 18 vote leads to independence. Lloyds Banking Group, which owns Halifax and Bank of Scotland, said it also had plans to set up new "legal entities" in England if the independence campaign succeeds.

The vote "could have a bearing on the bank's credit ratings and the fiscal, monetary, legal and regulatory landscape to which it is subject," RBS said in a statement.

Despite months of debate, there are still no clear answers about what currency an independent Scotland would use, how much U.K. public debt it would take on and who would regulate Scottish banks. Those issues, among others, would have to be settled in protracted negotiations before Scotland achieves full independence.

Alex Salmond, Scotland's leading politician, dismissed the banks' warnings as "scare-mongering" and accused the British government of exploiting the news for political gain. Salmond alleged the Treasury had breached financial rules because it briefed journalists about RBS's plans ahead of the market announcement.

Until recently, the prospect of an independent Scotland was dismissed as highly unlikely. But the latest polls suggest the independence campaign is gaining momentum. Almost 4.3 million people have registered to vote, the largest electorate ever for a ballot in Scotland.

That has sent jitters through the markets, prompting investors to sell off the British pound. The pro-independence campaign argues that Scotland could sustain itself economically, but opponents — including some businesses in Scotland and England — have focused on the uncertainties.

The currency debate is especially worrisome to Scotland's financial-services industry, which accounts for a quarter of its economy, excluding oil and gas. Salmond hopes Scotland could continue using the pound through a currency union with the rest of the U.K., but Britain's central government says that's not possible.

On Thursday, leading retailers including supermarket giant Asda and department store John Lewis also chimed in, raising alarms about possible higher prices for Scottish shoppers.

"It does cost more money to trade in parts of Scotland and therefore those hard costs, in the event of a yes vote, are more likely to be passed on," said John Lewis Partnership chairman Charlie Mayfield.

RBS, which employs 11,500 people in Scotland and is a symbol of its financial sector, said it would be necessary to move its holding company and its main operating entity, the Royal Bank of Scotland PLC, to England.

Despite its name, RBS is currently majority owned by the taxpayers of the entire U.K., since it needed a government bailout in 2008 at the height of the financial crisis. It has operations throughout the U.K. and foreign countries, but its relocation would be a symbolic blow to an independent Scotland.

The U.K. government also owns a significant share of Lloyds.

Smaller banks including Clydesdale, TSB and Tesco also announced similar contingency plans to minimize risks to their businesses.

The financial group Standard Life earlier said it was ready to move parts of its business to England in case of independence. Precautionary measures include transferring pensions, investments and other long-term savings to new companies to ensure they remain part of Britain's currency and tax regime.

Experts say the banks announced their contingency plans to avoid further risks to themselves or the markets.

"Given the huge amount of volatility next week in case of a yes vote, the Bank of England would have put high pressure on the banks to move south for the purposes of preserving financial stability and confidence," said Ronald MacDonald, a political economy professor at the University of Glasgow.

Some downplayed the warnings. Ian Gordon, an analyst at Investec, said the banks' warnings conformed to market expectations and do not signal any real changes.

"I wouldn't discount over time there being some broader implications, but it's fairly procedural," he said.

But Margaret Taylor, 56, from Dumbarton, near Glasgow, was having second thoughts.

"Everything I hear about food costing more and the banks moving away has certainly made me think it's not worth taking the risk," the shop worker said. "People tell me it's all bluff and scare stories, but what if it's not?"

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Sylvia Hui reported from London.