ALISTAIR Darling is facing mounting pressure to order the return of a £500,000 donation to the Better Together campaign from the head of the world's largest oil trading company.

The Labour former Chancellor and leader of the anti-independence campaign defended the contentious gift from Ian Taylor, the head of Vitol, and denounced "personal attacks" on Mr Taylor by the pro-independence lobby.

But in addition to mounting pressure from the SNP, Labour's own former first minister Henry McLeish raised questions about the wisdom of receiving the businessman's gift, and called for "absolute transparency".

He said: "We have to be very careful about what kind of money should be used. The Unionist campaign has got to respond to legitimate questions and the criticism that's been put forward."

Labour backbench MP John Mann has called on the Tories to return donations given to the party by Mr Taylor.

Angus Robertson, the SNP's Westminster leader, said that demand should extend to the Better Together campaign.

"If Alistair Darling hangs on to this cash, the No campaign will be tarnished," he said.

Mr Taylor and his donation to the pro-UK campaign hit the headlines in light of Vitol's controversial activities in Serbia, Iran, Iraq and Libya.

Yesterday, Steve Murdoch, a history professor at St Andrews University, branded the No campaign's acceptance of Mr Taylor's donation "shameful".

Mr Murdoch ran an agency that provided aid to Bosnia in the 1990s war there, and suggested the money should be given to orphans of victims of death squads in Croatia and Bosnia.

The Better Together campaign has rejected Nationalist calls to hand back its largest single donation to date, and accused the pro-independence forces of mounting a dirty tricks campaign, which they deny.

Mr Darling said: "Ian Taylor is a respected figure in Scotland and in the UK. He has made a big contribution to Scottish life, including a personal investment that has revived the Harris Tweed industry."

The former Chancellor said the businessman had made clear his reasons for supporting the cause of keeping Scotland in the UK.

He added: "We are pleased to have his support and we think people who care about Scotland should be able to speak out against the Nationalists without facing - personal attacks."

At the weekend, Blair Jenkins, leader of the Yes campaign, made reference to Mr Taylor without naming him, stressing anti-independence forces were "largely funded by a Tory donor in the south who is not even eligible to vote here".

Last night, Mr Robertson claimed Mr Darling's refusal to hand back Mr Taylor's donation and investigate the background to it was undermining the credibility of the No campaign.

"Henry McLeish is absolutely right when he calls for clarity about the type of finance that we want.

"This is a serious error of judgement by Alistair Darling. He has to take this opportunity to reverse his position, do the right thing and hand the money back pending a full investigation by the No campaign."

Vitol has made it clear, in response to reports that in 2001 it paid $1 million to a Serbian warlord to secure an oil deal with the regime of Slobodan Milosevic, that it did nothing illegal.

A company spokeswoman said it is confident its tax affairs are compliant with the law in all of the countries where it operates.