Section five of the constitution was explained as stating that the president's executive powers may be exercised thought the vice president, ministers, or officers in the public service.

'Unexpected development'

Andrew Simmons, Al Jazeera's Africa editor, said: "This is a significant and unexpected development.

"This effectively gives the all clear for the government to carry on as it is doing.

"This could be a pre-emptive strike against three separate cases which are all converging in one hearing tomorrow at the high court, in which human rights activists and opposition politicians are trying to get a ruling which takes away power from the president and gives it to Goodluck Jonathan as acting president.

"It sounds like semantics but it is very important. The term acting president gives power to Jonathan which the ruling party here doesn't want to happen."

Jonathan's office did not immediately comment and it was not clear whether he would act on the ruling.

The vice-president has been representing Yar'Adua at cabinet meetings and official functions but executive powers have not yet been transferred to him.

This has led to questions of the legality of government decisions and protests against there being no formal transfer of power.

'Meaningless'

The three cases to be heard in court on Thursday push for Yar'Adua to step down due to his ill health.

Femi Falana, a constitutional lawyer who filed one of the court challenges, said that Wednesday's ruling was "meaningless".

"Jonathan cannot act for Yar'Adua because no functions have been formally and directly delegated to him by Yar'Adua to enable him to act," he said.

Falana said that the constitution states that the president must formally explain his absence in writing to both chambers of parliament.

"When he does that and other processes as stipulated in the constitution have been carried out, then Jonathan can act as president," Falana said.