Larry Summers, a former top economic advisor to the U.S. government, has added his voice to the numbers calling on Scottish people to vote against independence in next week's referendum.
Ex-Treasury Secretary Summers, currently a Harvard professor, but formerly director of the White House National Economic Council and Harvard president, joined figures as diverse as Nobel-prize winning economist Paul Krugman and billionaire investor George Soros in urging the Scots to vote "no" on Thursday.
"It would be a grave mistake for Scotland to leave," Summers told CNBC.
"My expectation is that as people see what's really involved with Scottish independence and the very grave consequences that would follow, they will cast a vote for the future rather than a vote for frustration."
He added that he is now more optimistic about a "no" vote, after this week's polls suggested weakening support for the Yes campaign.
The economist was speaking at Yalta European Strategy, an annual conference dedicated to fostering closer ties between Ukraine and the European Union.
Ukraine's fraught relationship with Russia, and attempts to move closer to the EU, have sparked violent conflict, causing thousands of deaths. The EU announced a series of new economic sanctions against key Russian politicians and business leaders Friday, in an attempt to keep up the pressure on Russia to reach a peaceful solution with its neighbor.
"Ultimately, Ukraine's future is going to be determined by what happens with its economy. These last 20 years have not been easy years, and the conflict has greatly magnified the challenge," Summers said.
He called for "strong and comprehensive" support from the West, and suggested some planned loans to stricken Ukraine should be replaced with grants.
Ukrainians also have "to do what they have to do" to tackle corruption in the country, he added.
The dispute with Russia has raised the question of energy security for the EU, where Germany in particular relies on Russian gas supplies to power its industry.
The U.S. has to reinforce its own power by exporting its relatively newly-tapped shale gas reserves, according to Summers.
"There is no strategic opportunity for the U.S. more important than the opportunity to become a substantial exporter of both natural gas and energy," he said.
"There is no easier public policy question facing the U.S."
- By CNBC's Catherine Boyle. Reporting by Steve Sedgwick in Kiev.