Mr Bondevik became prime minister in October 1997 and heads a fragile three-party coalition government.

The prime minister's office announced on Monday that the Culture Minister, Anne Enger Lahnstein, would take over as acting prime minister.

'Depressive reaction'

"Mr Bondevik has been put on sick leave by a doctor, initially for a week from 30 August, because of a depressive reaction as a result of over-work," the statement said.

The 50-year-old is an ordained priest in Norway's state church.

He has been under pressure following a surge in interest rates and a plunge in the value of Norway's currency, the kroner, linked to weak oil prices and economic woes in Asia and Russia.

Mr Bondevik can call upon only 42 votes in the 165-seat parliament and has to work hard to win support from opposition parties on the left or the right to push through any legislation.

A planned trip to the Middle East from 7 to 12 September is also to be postponed.