Mitchell says it's time someone spoke out about what goes on in international business deals. Lindsey Mitchell has told Fairfax Media, The Huffington Post and the BBC's Panorama program that he bribed senior officials in the Gaddafi regime on behalf of, and while working for, Unaoil in mid 2009. He has also confessed to the FBI and British authorities, saying he will testify against Unaoil. Mitchell further says he worked alongside a Libyan businessman who was involved in the security contract to guard the US consulate in Benghazi. In September 2012, the embassy was attacked by members of extremist group Ansar al-Sharia, resulting in the death of US ambassador Christopher Stevens and one other diplomat. Mitchell claims the Libyan businessman was corrupt. The businessman denies the claim.

Unaoil's owners, the Ahsani family: Saman, Cyrus and Ata. The company denies paying bribes. While the world has been stunned by revelations flowing from large data leaks involving the Unaoil Files and the Panama Papers – both of which have prompted investigations by multiple policing agencies – what has been missing from these international scandals is a live witness. Mitchell's extraordinary decision to implicate himself and Unaoil in bribery is significant because it will bolster the case against Unaoil being built by the UK Serious Fraud Office, the FBI and the Australian Federal Police. US ambassador to Libya Christopher Stevens, who died in the 2012 attack on the US consulate in Tripoli. Credit:AP These agencies are seeking to prove that Unaoil was hired by so many large Western companies because the firm paid bribes to win contracts on their behalf.

Mitchell warns that the use by Western multinationals of local agents to win contracts in developing nations often involves bribery and corruption. Whistleblower Lindsay Mitchell has confessed to the FBI and British authorities and is willing to testify against Unaoil. THE LIBYA DEAL Mitchell joined Unaoil in early 2009 after an interview with the company's owners, the wealthy Ahsani family of Monaco. By then, Unaoil was working as an agent in corruption-prone oil-producing countries for many leading multinationals, including Halliburton; Kellogg Brown & Root; Honeywell; FMC; Petrofac; and Samsung. In 2010, Unaoil was hired by Australia's Leighton Holdings to win Iraq oil pipeline contracts worth more than $1 billion.

Unaoil denies it bribed foreign officials, politicians, oil ministers and others to win deals for its multinational clients. But Mitchell says that during his first few weeks with Unaoil, he suspected something was amiss when he observed chief executive Cyrus Ahsani handling a contract for engine-maker Rolls-Royce. Mitchell recalls: "Cyrus Ahsani had to rush off to the Swiss Alps that weekend to close a deal involving Rolls-Royce very quickly. I heard him ask Ata [Ahsani] that he is going to need cash. That is when I realised these boys [the Ahsanis] were starting to do some underneath-the-table stuff to make things happen." Unaoil or its agents have represented Rolls-Royce over a decade in several countries, including Iraq, Azerbaijan, Kazakhstan and Angola. Rolls-Royce is now being investigated by the British Serious Fraud Office. Unaoil wanted Mitchell, who was previously chief executive of publicly listed Canadian oil services company Western Energy Services Corp, to win favour from Gaddafi regime officials he knew from his time working in the Middle East.

One evening in August 2009, he was called to a late-night meeting at a coffee shop on the outskirts of Tripoli by one of those officials – a manager at a Libyan National Oil Company subsidiary. Mitchell says the man made it clear that he and two other oil officials could help Unaoil's clients succeed in Libya. In a car park, the official then passed him an envelope filled with confidential tender documents relating to a $45 million Libyan government contract. Mitchell says he briefed his bosses at Unaoil, the Ahsanis, about the late-night meeting and explained that the official would "want to be paid for his efforts". "They said, fine, give him whatever he needs," Mitchell recalls he was told by the Ahsanis. A short time later, one of the Ahsanis' relatives in Libya passed Mitchell an envelope filled with cash to pay off three officials. "I would say there would have been $US15,000 to $US20,000 in that envelope," he says.

Mitchell claims he delivered the cash-filled envelope to the oil official's home in Tripoli close to midnight. However, he says the bribe payment immediately began to play on his conscience. He had witnessed plenty of corruption in the oil industry, but had never been the one to hand over the cash. "I sat back after that night and thought, 'If my kids knew what I did, they would totally disrespect me.' " A few days later, Mitchell told Unaoil he was resigning. When approached for comment, a Unaoil spokeswoman said: "Although there is a great deal that the company would like to say about the subject matter of this article, there is an investigation ongoing.

"Any allegations of corruption are denied. It would be inappropriate to make further comments at this juncture but the company looks forward to speaking out on this in the fullness of time." CONSULATE TRAGEDY Mitchell has also provided new insight into the Libyan businessman involved in the security contract at the US consulate in the city of Benghazi. Mitchell worked with the Libyan agent, Muhannad Alamir, who was involved in the private security contract to guard the consulate. In 2012, the building became the site of a protest that turned violent, during which the American ambassador and an information officer were killed.

Fairfax Media does not suggest the involvement of Mr Alamir in the contract had anything to do with the attack on the embassy and the death of the ambassador. Mr Alamir had worked with Unaoil and Mitchell in Libya prior to Mr Alamir's involvement in the Benghazi contract in 2012 and, according to Mitchell, was party to corrupt business dealings alongside Unaoil in 2009. Mitchell says he is "shocked" that a US State Department private security contract to guard the American embassy was later issued to another firm represented by Mr Alamir in Libya. Mitchell made himself known to the FBI and Serious Fraud Office after the Unaoil story broke in March, and has given statements to both agencies. Loading

He says he is prepared to testify about his knowledge of the companies he has worked for before any government and law enforcement inquiries. "I'm 57 now and my career is coming to end. It's time someone spoke out about what really happens in some international business deals," he says.