Mr Fjell lost the confidence of the board

The resignation of Olav Fjell came after a marathon overnight board meeting lasting into Tuesday, and follows the departure of chairman Leif Terje Loeddesoel at the weekend.

Just a week earlier, the Statoil board had expressed its confidence in Mr Fjell, who has come under fire for a $16m package of incentives paid to consultants linked to Iran.

Norwegian police suspect the incentives were effectively bribes, aimed at winning contracts in Iran.

'A calmer atmosphere'

"This is a sad day," said Statoil chief financial officer Inge Hansen, who takes over as interim boss.

"I am going to work hard, and work to get the employees with me."

In a statement, the company said it was "important that a calmer atmosphere be established as quickly as possible in relation to the group, so that Statoil's development can continue."

Statoil would not say what new information led to Mr Fjell's decision to step down, so soon after being supported by the board.

The apparent change of heart has led to reports of a split in the board, with some members unsatisfied at the level of disclosure Mr Fjell made at the earlier meeting.

Ups and downs

Statoil, which was founded in 1972 and partly privatised in 2001, has a history of management turbulence.

The government sacked the board in 1999, and the chief executive resigned soon after in a dispute about cost overruns.

His predecessor had also been ousted for failing to control costs.

Statoil's purpose is to explore internationally for oil and gas, as a hedge against domestic supplies running out in Norway, currently the world's third-biggest petroleum exporter.