THE pro-union Better Together campaign is facing mounting pressure to return its largest donation, after it emerged that the businessman who gave it heads a company with a web of operations in tax havens.

The anti-independence group has been under fire from the SNP since it declared a week ago that it had accepted £500,000 from Ian Taylor, a Conservative Party backer who is president of the world's largest oil trader, Vitol.

Accounting for almost half the £1.1 million given to Better Together since its launch last June, the donation came after Taylor met the campaign's leader, former Labour Chancellor Alistair Darling.

Darling yesterday broke cover and spoke out in defence of Taylor. He also said Better Together would be keeping the donation.

Although the donation was legal, the SNP has urged Better Together to return it on ethical grounds because of controversial Vitol activities in Serbia, Iran, Iraq and Libya.

Financial statements filed in Luxembourg last month by a key Vitol company, Vitol Holding II SA, revealed multiple operations in tax havens. Covering the year to December 31, 2011, when Vitol Holding II SA recorded a pre-tax profit of $2 billion, the Consolidated Financial Statements list 90 companies and 100%-owned subsidiaries which comprise the "Vitol Group".

Of these, 23 were in well-known tax havens: 16 in Bermuda, five in the British Virgin Islands, one in Panama, and one (90% owned), in the Arab Emirate of Fujairah.

One subsidiary is Pipsa Energy (Bermuda) Ltd. Ex-Labour MP and energy minister Brian Wilson was a paid adviser to Pipsa in 2004-05.

Vitol has previously admitted paying £1m to the Serbian warlord Arkan in the 1990s to expedite an oil deal, but insists it broke no laws. Arkan was responsible for crimes including directing ethnic cleansing and murder during the Yugoslavian wars before his death in 2000.

In 2007, Vitol pled guilty to grand larceny in a deal with the Manhattan District Attorney in the US after paying what the firm called "surcharges" to Iraqi officials in Saddam Hussein's regime under the United Nations oil-for-food scheme. The DA's office described the payments as "kickbacks".

In 2010, the US Commodity Futures Trading Commission (CFTC) fined two Vitol subsidiaries $6m for "wilfully failing to disclose material facts" to the New York Mercantile Exchange for almost two years. Vitol neither admitted nor denied the CFTC's findings, but put in new compliance measures.

After a week's silence, Darling yesterday defended Taylor – who has a key stake in the Harris Tweed industry. Darling said: "Ian has made clear his reasons for supporting Scotland as a strong part of the UK - we think that people should be able to speak out against the nationalists without facing this kind of personal attack."

SNP Westminster leader Angus Robertson said the credibility of Darling and the No campaign was damaged by keeping the donation.

Better Together declined to comment on Vitol's operations but said Taylor was a respected figure in Scotland and the UK.

A Vitol spokeswoman said the firm was confident it was fully compliant with all relevant regulations globally, and that "all its tax affairs are compliant with the appropriate legislation in each of the jurisdictions in which it operates".