Goodluck Jonathan, Nigeria's vice-president, has been empowered by the country's parliament to take over as head of state in the absence of the nation's ailing president.
Both houses of the national assembly on Tuesday voted to install Jonathan as acting leader, until Umaru Yar-Adua declares he is fit enough to return.
"The vice-president ... shall henceforth discharge the functions of the office of the president, commander-in-chief of the armed forces of the federation, as acting president," the Senate motion said.
But it was not immediately clear if the parliamentary vote had legal sanction, with no provisions in the constitution for the national assembly to take such a step.
'Coup plot'
Festus Keyamo, a constitutional lawyer, said the move was illegal.
"What they have done amounts to a coup plot. It is a desperate decision by desperate politicians who were trying to save their face," Keyamo told the AFP news agency.
Nigeria's constitution says the president must make a written declaration that he is on vacation or unable to carry out his duties before a transfer of power can take place.
Yar'Adua has been receiving treatment for a heart condition in Saudi Arabia since last November.
The senate said it based its decision on a January 12 interview with the BBC, in which the president said he would return to work once his doctors cleared him.
"We came to the conclusion that the president, through his declaration transmitted worldwide on the BBC, has furnished this parliament with irrefutable proof that he is on medical vacation ... and has therefore complied with the provision of section 145 of the 1999 constitution," David Mark, the senate president, said.
"The last 78 days have been very challenging to us as a nation ...however we have examined all options available to us and today rightly concluded it is necessary to take this stance to allow this country to move forward," he said.
Yar'Adua's absence has caused a cease-fire with fighters in the oil-rich Niger Delta to unravel and had left no one formally in charge of the nation of 150 million.
Al Jazeera
