Pressure is growing for the firm's chief executive Olav Fjell to resign, after Leif Terje Loeddesoel decided to quit late on Monday night.

Mr Loeddesoel stepped down from his job at Statoil after a police investigation linked to the firm's activities in Iran.

The controversy centres on a $15m fee paid to a London-based group of consultants that police suspect could be a bribe to win contracts from Tehran.

Last year, the state-owned Norwegian firm won a contract to operate one of the largest gas fields in Iran, South Pars.

"It's obvious that the company's reputation has been weakened," Norway's Oil and Energy Minister Einar Steensnaes told Norwegian radio.

He said it was up to Statoil's board to decide if Mr Fjell should resign.

Corruption

Statoil, which is owned by the Norwegian government, is an important part of Norway's economy.

Mr Fjell's four years as Statoil boss have been marked by an ambitious strategy of expanding into politically risky parts of the world.

This has involved investments not only in Iran's South Pars gas field in the Gulf - the first step in a push into the Middle East - but in Angola and Venezuela as well.

But it is the Iran deal which has proved the most controversial after it emerged that Statoil paid $15.2m to Horton Investment, a London-based firm owned by an Iranian called Abbas Yazdi.

The contract was revoked earlier this month, but is now the centre of the police investigation to see whether "punishable corruption" has been committed.

Oil companies are coming under increasing pressure from some investors as well as campaign groups to be open about deals with government and government-linked "facilitators."

Groups such as Transparency International point to massive corruption in oil-rich states such as Nigeria and Angola, where the International Monetary Fund suspects $1bn a year may be going missing from oil revenues.