The SNP has called on Better Together to return a £500,000 donation following reports that donor Ian Taylor's company is in negotiations with a Russian oil firm.
Mr Taylor, chief executive of the oil trading firm Vitol, gave the money in a personal capacity in support of a No vote in the independence referendum.
It has been reported that Vitol is negotiating a finance deal with Kremlin-controlled Rosneft, which has been targeted by US sanctions against President Vladimir Putin.
Vitol declined to comment because the company "never comments on client relationships" and states that it will remain "fully compliant with all applicable international laws and regulations".
SNP Westminster leader Angus Robertson said: "It is simply unbelievable that the No campaign is happy to accept donations from people like Ian Taylor."
A Better Together Spokesman said: "These comments from the SNP are a revealing insight into the type of Scotland they want to create.
"Anyone who disagrees with them is subjected to personal attacks.
"Any and all opposition must be silenced."