The campaign battle over Scottish independence took a bitter turn on Saturday when a senior nationalist warned businesses such as BP that they could face punishment for voicing concern over the impact of secession.

With the fate of the United Kingdom on a knife edge, the economic future of Scotland has become one the most fiercely debated issues less than six days before Scots decide on whether to break away.

Nationalists accuse British Prime Minister David Cameron of coordinating a scare campaign by business leaders aimed at spooking voters while unionists say separation is fraught with financial and economic uncertainty.

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But former Scottish Nationalist Party deputy leader Jim Sillars went much further than separatist leader Alex Salmond, warning that BP's operations in Scotland might face nationalisation if Scots voted for secession on Sept. 18.

"This referendum is about power, and when we get a 'Yes' majority we will use that power for a day of reckoning with BP and the banks," Sillars, a nationalist rival of Salmond's, was quoted by Scottish media as saying.

"BP, in an independent Scotland, will need to learn the meaning of nationalisation, in part or in whole, as it has in other countries who have not been as soft as we have been forced to be," Sillars said.

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When asked about the comments in a BBC radio interview on Saturday, Sillars confirmed he had raised the prospect of nationalisation but said he had used the term to get media coverage and that nationalisation was not on the table.

A spokesman for BP declined to comment. A spokeswoman for the Scottish nationalists could not be reached for comment.

The boss of BP, Bob Dudley, has said that Scottish independence could cause his company "uncertainties" and that he did not want to see Scotland drifting away.

"The heads of these companies are rich men, in cahoots with a rich English Tory Prime Minister, to keep Scotland's poor poorer through lies and distortions," Sillars was quoted in Scottish media as saying. "The power they have now to subvert our democracy will come to an end with a 'Yes'."

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In an extraordinarily strong attack on business, Sillars also said banks such as Standard Life would face tougher employment laws after a vote for independence.

Major banks, oil companies and supermarkets have said that a vote for secession would create concern: North Sea oil would have to be divided up while there is uncertainty over the future currency and central bank of an independent Scotland.