LONDON: European refiner Varo Energy, owned by the world's biggest independent oil trader Vitol, U.S. private equity firm Carlyle and Dutch firm Reggeborgh Invest, has been asked to collaborate with a probe into British biodiesel group Greenergy.

Britain's Serious Fraud Office said on Thursday it had opened a joint investigation with Dutch authorities into aspects of biodiesel trading at Greenergy and third parties.

"Varo has indeed been asked to collaborate in an ongoing investigation. As a matter of policy, Varo always cooperates fully with the relevant authorities in any jurisdiction in which it operates," a Varo spokeswoman said when asked about the probe.