Alan Booth, currently a non-executive director, has been named interim CEO and executive director, Ophir said.

The company is looking "to rebalance its portfolio towards a larger production and cash flow base, to support more focussed and sustainable exploration activity," the board said in a statement.

Cooper served as chief executive for almost seven years. He led the company's IPO in 2011, its sale of part of its interest in Tanzania in 2013 and its acquisition of Salamander in 2015.

Most recently, Cooper initiated a proposed acquisition of Santos Ltd Southeast Asian assets.

However, the company has struggled to secure $1.2 billion (889 million pounds) in bank loans needed to fund the Fortuna deepwater liquefied natural gas (LNG) project in Equatorial Guinea, forcing it to push back first production to 2022.

This week, Equatorial Guinea's energy minister told Reuters that Ophir would be forced out of the project unless loan deals were presented to the government by December. Gabriel Obiang Lima said the country had already lined up a potential replacement.

Ophir's board said it would continue to focus on realising value for shareholders from the Fortuna project and work with Equatorial Guinea on achieving that.

However, Morgan Stanley analyst Sasikanth Chilukuru said in a note that Cooper's departure "adds more uncertainty to the investment thesis, one that is dominated by the Fortuna FLNG project".

Ophir shares fell as much as 8 percent of Friday and stood down 3.2 percent at 63.70 pounds at 1014 GMT.

The stock is down 33 percent in the past year, losing out on gains enjoyed by peers courtesy of a rise in energy prices including Brent crude oil and spot LNG .

(Reporting by Arathy S Nair in Bengaluru and Oleg Vukmanovic in London; editing by Jason Neely)