A senior British executive at an oil and gas firm owned by a major Tory donor has pleaded guilty to 11 counts of bribery.

David Lufkin, 51, the former global head of sales at Petrofac, admitted offering corrupt payments in an attempt to secure contracts in Saudi Arabia worth $3.5bn (£2.7bn) and contracts in Iraq worth $730m (£566m).

Petrofac shares fell by 20% in response to the announcement, according to Reuters.

Petrofac’s owner and chief executive, Ayman Asfari, is a major financial backer of the Conservatives. Together with his wife he has donated almost £800,000 to the party since 2009.

In 2014, the then prime minister David Cameron appointed Asfari to be a “business ambassador”. The ambassador’s group, which was intended to “promote the UK’s excellence internationally” and secure inward investment into the UK, was closed last month.

The Serious Fraud Office announced it was investigating Petrofac over allegations of bribery, corruption and money laundering in May 2017. Asfari was arrested and questioned under caution by the SFO, along with a member of Petrofac’s senior management team.

The investigation was spun off from a previous inquiry into the activities of the Monaco-based oil and gas services firm Unaoil, which has denied any wrongdoing. Both investigations remain ongoing.

According to the SFO, the charges against Lufkin included payments of $2m to two agents in respect of a $330m contract in the Badra oilfield in Iraq, which Petrofac secured in 2012. Attempts to vary and extend the contract with offers of corrupt payments were unsuccessful.

Separate payments of $4m were made to secure a $400m contract at the Fao terminal project in Iraq. The contract was awarded in 2012 and extended in the three following years.

Approximately $45m in corrupt payments were made to Petrofac’s agent in Saudi Arabia between 2012 and 2015 for contracts worth $3.5bn.

Petrofac said no charges had been brought against the company, but that several individuals and corporate entities were alleged to have acted together with Lufkin. It added that no current board member of Petrofac Ltd was alleged to have been involved.

In a statement, Petrofac’s chairman, René Médori, said: “The SFO has chosen to bring charges against a former employee of a subsidiary company. It has deliberately not chosen to charge any group company or any other officer or employee.

“In the absence of any charge or credible evidence, Petrofac intends as a matter of policy to stand by its employees.

“Petrofac has policies and procedures in place designed to ensure that we operate at the highest levels of compliance and ethics.”

Lufkin will be sentenced at a later date.