Robin McAlpine, director of the left-wing Jimmy Reid Foundation think-tank, said most Scots who wish to remain in the UK would want to see the cash returned.
He said: "Better Together is developing a reputation for speaking on behalf of the business elite and its interests. Its refusal to even consider whether this donation sends the wrong message is not helping one bit."
Mr McAlpine spoke out as pressure continued to mount over a £500,000 donation from Ian Taylor. The SNP has called on Alistair Darling, the head of Better Together, to return the money after details emerged of controversial Vitol business deals in Serbia, Iran and Iraq.
Better Together has dismissed calls to return the cash and accused the Nationalists of mounting a smear campaign over the donation.
It again defended accepting the money. A spokesman for the campaign said: "We would no sooner be asking Ian Taylor to take his money back from Better Together as we would ask him to take back his substantial investment in Harris Tweed.
"Ian Taylor's financial backing revived the industry, safeguarded jobs and is helping it to become one of Scotland's best recognised and most successful global exports."
The spokesman said the campaign was pleased to have the support of "someone who has made such an important contribution to Scotland".
Vitol has made it clear, in response to reports that in 2001 it paid $1 million to a Serbian warlord to win an oil deal with the regime of Slobodan Milosevic, that it did nothing illegal.
A Vitol spokeswoman has stressed it is confident its tax affairs comply with the laws in all countries in which it operates.