Australian police have flown to London to work with British authorities on the international "bribery, corruption and money laundering" case involving oil services firm Unaoil and several leading multinational companies. 
Fairfax Media can also reveal that several overseas witnesses have agreed to co-operate with investigators to corroborate evidence showing Unaoil paid kickbacks to help oil industry firms win large contracts in several countries.
On Wednesday, Britain's most powerful anti-corruption agency released a statement that it was investigating Unaoil for suspected "bribery, corruption and money laundering".
The revelation comes after a Fairfax Media investigation into Unaoil's payment of kickbacks to win contracts for multinational companies across the world.
The public statement by the Serious Fraud Office that it is investigating Unaoil and its "officers, its employees and its agents" for serious criminal activity will alarm dozens of companies that used Unaoil as a consultant or agent in such countries as Iraq, Libya and Iran. It is the first time a police agency has publicly commented on the nature of investigations into Unaoil, which involve the SFO, the Australian Federal Police and the US Department of Justice and the FBI.
While the SFO has not named any of Unaoil's clients, investigators are likely to examine Unaoil's work for the offshore arm of Australian company Leighton Holdings (now trading as CIMIC), British companies Rolls-Royce and Petrofac, and several leading European and American companies.
One source close to Unaoil's owners, Ata Ahsani and his sons Cyrus and Saman, told Fairfax Media that the trio were in "full-on panic mode". "They have been calling around trying to find out who is talking," the source said.
The SFO statement says the agency is probing "suspected offences of bribery, corruption and money laundering" by the Monaco-based company. It also revealed it had been approached by "a number of sources who may have information relevant to this investigation", inviting others to come forward through its secure and confidential reporting channel.
Witnesses told Fairfax Media that investigators had travelled through Europe 10 days ago, meeting Unaoil insiders.
The SFO is a specialist prosecuting authority which, its website says, tackles "the top level of serious or complex fraud, bribery and corruption". It takes on "a small number of large economic crime cases".
British investigators were working closely with the AFP. A well-placed British source said Australia's contribution to the Unaoil probe had been "immense" and was evidence of how anti-bribery investigators were increasingly working across borders and agencies.
Fairfax Media and the Huffington Post reported in a series of exposes earlier this year that Unaoil had bribed officials and businessmen, seeking corrupt influence in the oil industry in dozens of countries.
The company has denied any wrongdoing, while refusing to answer detailed questions about its behaviour. It has threatened to sue Fairfax Media for defamation, although it has not issued a writ.