LEADING political figures behind Better Together, the pro-Union campaign, are maintaining a wall of silence over a £500,000 donation from a controversial business figure.

Former Chancellor Alistair Darling, now head of Better Together, and Shadow Foreign Secretary Douglas Alexander – a critic of Ian Taylor's donations to the Conservatives when they attracted accusations of cash-for-access – have both been unavailable for comment.

Their silence came despite the SNP's Westminster leader Angus Robertson ramping up the pressure and writing to Mr Darling on the issue of the donation from Mr Taylor, the president and chief executive of oil trader Vitol.

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While Mr Taylor was criticised by Labour's Westminster backbencher John Mann and by Mr Alexander for being invited to dine at Downing Street following his Tory donations, it is Vitol's controversial activities in Serbia, Iran, Iraq and Libya that have led to questions being asked about the businessman's personal backing for Better Together.

The pro-Union campaign has strongly rebuffed all suggestions that the donation from Mr Taylor is inappropriate and accused the SNP of masterminding a smear campaign. A spokesman for Better Together said last night: "I think we have made our position clear on this. We certainly will not be returning the donation."

But Mr Robertson pointed out that Mr Mann, a member of the House of Commons Treasury Select Committee, was reported last September to be calling on the Tories to return donations from Mr Taylor.

The Daily Telegraph reported: "Vitol was accused of 'immoral' trade and 'backing corrupt regimes' by John Mann MP, a Labour member of the Treasury Select Committee, who demanded that the Tory party hand back the 'dirty money' it has received from Mr Taylor."

Mr Robertson said: "Presumably, Alistair Darling agrees with his Labour Westminster colleague, John Mann MP.

"If Mr Darling does agree that the Tories should hand their donations back, he cannot possibly believe it is right for the No campaign to hold on to their half-a-million pounds from Mr Taylor – who is the No campaign's principal donor."

Mr Robertson said this was why he had now written to Mr Darling calling on him to hand this donation back pending a full internal investigation into the circumstances of the donation, and a pledge to make the findings public.

Vitol has admitted paying $1m (US) to Serbian warlord Arkan in relation to an oil deal but insisted it broke no laws. Most recently, it admitted involvement in tran-shipping Iranian oil but stressed this was before the introduction of all relevant international legislation and this did not break any sanctions.

The company was one of the first to get involved in the Libyan oil trade while the war was still going on there. The UK Government admitted International Development Minister Alan Duncan, a former consultant to Vitol and friend of Mr Taylor, was part of Whitehall talks.

Vitol, which has been threatening legal action against The Herald and some Scottish websites this week, strongly rebuts the suggestion Mr Duncan was involved in its Libyan deals.

A spokesman for Vitol said neither Mr Duncan nor the Foreign Office brokered the transactions in question. The spokesman also said Vitol was satisfied that all its tax affairs were compliant with the appropriate legislation.