Human rights campaigners including the actress Mia Farrow and three winners of the Nobel Peace Prize have called on UBS to pull out of PetroChina’s $5 billion (£2.4 billion) Shanghai flotation over the oil giant’s links with Sudan. In a letter seen by The Times, more than a dozen international luminaries have attacked the Swiss banking group for its role as a lead underwriter on the forthcoming initial public offering.

PetroChina, Asia’s biggest oil producer, is 88 per cent-owned and controlled by the state-run China National Petroleum Company (CNPC), the largest player in Sudan’s oil and gas industry.

The letter urges UBS to ask CNPC either to suspend its activities in the country or implement a corporate governance policy to “mitigate the impact of its operations on innocent civilians in Sudan”. It adds: “Should UBS find CNPC unresponsive, they should withdraw from the public offering.

“It is time for UBS to speak out and take action today as tomorrow will be too late for the people of Darfur.”

The move comes days after Warren Buffett, the investor, sold the last of his shares in PetroChina, which is already listed in Hong Kong.

Fidelity Investments cut its stake in the group in May under pressure from campaigners furious over atrocities in the Darfur region.

Two thirds of Sudan’s oil is bought by China every year in a deal estimated to be worth up to $4 billion to the government in Khartoum. Critics claim the agreement has helped to protect the Sudanese economy from the full effects of international pressure over the conflict in Darfur.

This year the US strengthened sanctions imposed a decade ago, amid continuing allegations that Khartoum is funding the Janjawaid militia operating in the war-torn region.

UBS yesterday insisted it did not have a case to answer as PetroChina had “no operations in or dealings with” Sudan. However, the Aegis Trust, a genocide prevention company, countered that PetroChina and CNPC were “to all intents and purposes the same business”. The American-based Sudan Divestment Fund this year claimed that the two companies share many of the same executive directors and have a noncompetition agreement. It also claimed that PetroChina is building a refinery to process Sudanese oil.

Wang Guoliang, PetroChina’s chief financial officer, said two years ago that the company had an option to buy CNPC’s operations in Sudan. He was appointed to his current role in April.

As well as Ms Farrow, the letter to UBS has been signed by Jody Williams, winner of the Nobel Peace Prize in 1997 and former head of the UN Human Rights Council Mission on Darfur this year.

Other signatories include Betty Williams, recipient of the Nobel Peace Prize in 1977, and Justice Richard Gold-tone, the former chief prosecutor for the International Criminal Tribunals for Yugoslavia and Rwanda. Elie Wiesel, the Holocaust survivor and winner of the Nobel Peace Prize in 1986 signed up to the campaign on Thursday night.

Nick Donovan, head of campaigns at the Aegis Trust, said: “While the international community condemns [Sudan’s] President Bashir with words, oil revenue from PetroChina/CNPC is insulating the Government from political pressure. UBS’s actions send the wrong signal at the wrong time.”

PetroChina hopes to raise $5 billion from the sale of yuan-denominated A shares on the Shanghai stock market. The listing is expected to take place next month with the investor road-show due to begin next week.

PetroChina’s shares are traded as dollar-denominated H shares in Hong Kong and on the New York Stock Exchange as American depositary receipts.

The funds have been earmarked for a string of investments in China, including a production increase at the Daqing oilfield.

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