You can add former Federal Reserve Chairman Alan Greenspan to the list of those who think a decision by Scotland to declare independence from Great Britain would be a major mistake.

Scotland's referendum on independence is Thursday, and polls indicate it will be a close race.

The economic ramifications of independence would be "surprisingly negative for Scotland, more so than the Nationalist party is in any way communicating," Greenspan told the Financial Times.

"Their [Nationalist] forecasts are so implausible they really should be dismissed out of hand."

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Nationalists are wrong to claim that the United Kingdom would assent to a currency union with Scotland, Greenspan said.

"There’s no conceivable way the Bank of England is going to sit there as a lender of last resort to a new Scotland," he said.

Hedge fund legend George Soros, chairman of Soros Fund Management, also is worried about Scotland leaving the United Kingdom.

"This is the worst possible time for . . . Scotland to break with Britain," he wrote in the FT.

The Scots may have major policy disagreements with the rest of the United Kingdom. "But Scotland would be better placed to attain its political goals as part of a united Britain," he said.

Meanwhile, Harvard economist Ken Rogoff says the aftermath of Scotland's vote won't be pretty whatever the outcome.

"It's certainly a disaster for Scotland. First and foremost, it's going to be a horrible adjustment," he told CNBC.

"Even if it doesn't pass, people are not going to want to invest there, because they might do it again. People will migrate out of there."

The European Union will suffer too, as other autonomous regions, such as Catalonia in Spain, are inspired to seek independence, Rogoff said.

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Related Stories:

• Soros: 'Worst Possible Time' for Scotland to Quit the UK
• Harvard's Rogoff: Referendum 'Certainly a Disaster for Scotland'