Much to the surprise of the Norwegian media - which has been predicting his departure for days - Olav Fjell is staying in the top job.

But following a marathon board meeting on Monday night, Statoil's directors said Mr Fjell's Iran deal, now the subject of a police investigation, "should not have been entered into."

"The board criticises the chief executive because the assessments which were made in this connection were not good enough," the board said in a statement.

"The group can succeed internationally without becoming involved in activities which are in a borderland with regards to ethical norms and rules."

The announcement saw Statoil's share price rise 1.5%, having slid 2.6% on Friday as news of the investigation broke.

On the edge

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, thanks to a consultancy arrangement with Mehdi Hasemi Rafsanjani, son of Iran's former president.

That relationship stemmed from a contract worth $15.2m with Horton Investment, a London-based firm owned by another Iranian called Abbas Yazdi.

The contract was revoked on Friday, but is now the centre of the police investigation to see whether "punishable corruption" has been committed.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