Former First Minister Henry McLeish has voiced concerns over a £500,000 donation to the campaign to keep Scotland in the UK from oil trader Ian Taylor.
He told a Sunday newspaper there needs to be “absolute transparency” from the Better Together group about the money given to it by Mr Taylor, president and chief executive of Vitol.
His donation was revealed last weekend and the SNP immediately raised questions about the multinational firm, after allegations about deals in Iraq and the former Yugoslavia, disputed by the firm.
Vitol has acknowledged making some payments to Saddam Hussein’s regime outside the scope of the UN’s oil-for-food programme but rejects claims they were anything other than surcharges demanded by the state oil company.
The firm was accused of being linked to an oil deal involving Serbian criminal Arkan, who led paramilitary forces in the former Yugoslavia in the 1990s. But Vitol rejects any suggestion of illegal conduct and says it was not investigated by any government or authority.
Mr Taylor gave about half of the £1.1 million raised so far by the official No campaign group. Former Scottish Labour leader Mr McLeish told the paper: “We have to be very careful about what kind of money should be used (in the campaign).
“The unionist campaign has got to respond to legitimate questions and the criticism that’s been put forward.”
Alistair Darling, head of the Better Together campaign, said: “For our part, we are pleased to have (Mr Taylor’s) support and we think people who care about Scotland should be able to speak out against the nationalists without facing these kind of personal attacks.”