LONDON (Reuters) - A former Petrofac executive has pleaded guilty to eleven counts of bribery in an ongoing investigation, the Serious Fraud Office (SFO) said on Thursday, triggering a 20 percent fall in the British oil and gas firm’s shares.

David Lufkin, ex-global head of sales for Petrofac International Ltd, was convicted over corrupt attempts by Petrofac to secure deals worth more than $730 million in Iraq and more than $3.5 billion in Saudi Arabia, the SFO said.

The 51-year-old Briton will be sentenced at a later date. His lawyer did not immediately respond to a request for comment.

Petrofac said a number of Petrofac individuals and entities were alleged to have acted “with the individual concerned”, although no current board member of Petrofac Ltd is alleged to have been involved and there had been no further charges.

“The SFO has chosen to bring charges against a former employee of a subsidiary company,” Petrofac’s Chairman René Médori said in a statement.

“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’s shares were 21.1 percent lower at 440.55 pence at 1007 GMT.

The SFO said its investigation into Petrofac’s use of agents in jurisdictions including Iraq and Saudi Arabia is ongoing.

The SFO first said it had begun an investigation into Petrofac in 2017 as part of a wider probe into Monaco-based oil and gas consultancy Unaoil.

Petrofac said in February 2018 that it expected its top management to be interviewed as part of the SFO investigation.