ABUJA – A Nigerian court rejected on Friday a demand by top lawyers that a caretaker head of state be appointed until ailing President Umaru Yar’Adua returns from hospital treatment in Saudi Arabia.

Federal High Court judge Dan Abutu ruled there was nothing illegal about Yar'Adua's failure to write to parliament about his absence when he left for treatment on November 23.

Nigeria's constitution states that the vice president takes over in an acting capacity whenever the president informs in writing the leaders of the senate and house of representatives that he is going on vacation or is unable to carry out his duties.

Yar'Adua did not formally inform the legislature, forcing the Nigerian Bar Association (NBA) to go to court in a bid to have the vice president's powers extended to encompass those of an acting head of state, on grounds of a breach of the law.

Abutu however declared that "failure to transmit a written declaration to the national assembly before proceeding on vacation is not unconstitutional".

"There is no mandatory requirement for the president to make a transmission to the leadership of the national assembly before proceeding on vacation or treatment outside Nigeria," he said.

The 58-year-old president is receiving treatment for a serious heart condition in Jeddah and his absence has raised fears of a constitutional and political crisis in Africa's most populous country.

The court said because Yar'Adua did not formally declare his absence does not follow that his office is temporarily inactive.

"The vice president cannot validly discharge the function of the president under the constitution as acting president" on grounds that Yar'Adua did not inform the national assembly of his absence, said the court's most senior judge.

"The functions of the office of the president cannot be held in abeyance on account of the omission of the president to transmit a written declaration," said Abutu.

The NBA was disappointed with the ruling.

"We express our dissatisfaction with the judgement because as far as we are concerned ...the constitution places a duty on the president and not discretionary as ruled by the court," NBA's lawyer Amaechi Nwaiwu said.

The senate this week demanded that Yar'Adua write the required letter to parliament to pave the way for the transfer of power to Vice President Goodluck Jonathan.

The NBA case is the second of three separate suits before the Federal High court seeking to avert a political crisis in the country, which has a history of military coups.

A week ago the same court ordered the cabinet to decide within 14 days if Yar'Adua was fit to stay in office in view of his protracted hospitalisation.

The government -- all appointed by Yar'Adua -- declared days later that the head of state, in hospital since November 23, is capable of discharging his functions.

Jonathan on Thursday sought to douse mounting concerns as world powers began applying pressure, saying the president would return home soon. He did not give a date.

Jonathan has been filling in for Yar'Adua but does not possess full executive powers to act as head of state.

Yar'Adua's prolonged absence has stirred unease internationally.

"Nigeria is going through a period of uncertainty because of the prolonged illness of President Musa Umaru Yar'Adua," the United States, European Union, Britain and France said in a statement on Thursday.