The official result of the presidential election, held on April 21, gave Umaru Yar'Adua a crushing victory over his two nearest rivals who immediately challenged it, alleging vote-rigging and intimidation on a huge scale.

European Union observers said the polls were not credible and "fell far short of basic international standards". Yar'Adua has acknowledged flaws but said he has a popular mandate to rule the country that is Africa's biggest producer of crude oil.

A special tribunal sitting in the capital Abuja was scheduled to receive final submissions from lawyers and set a date for ruling when it resumed work on Monday after a seven-week break. Instead, it immediately adjourned to February 4.

The reason for the delay was a ruling by the Supreme Court last week allowing former Vice-President Atiku Abubakar, one of the two challengers, to question the electoral commission.

The tribunal last year stopped Abubakar from questioning Maurice Iwu, head of the Independent National Electoral Commission, but the Supreme Court said his questions were relevant and should be answered.

"The court agreed with us that we should not adopt the final addresses today but wait until Iwu has answered the interrogatories as directed by the Supreme Court," said Alfred Kasumu, one of Abubakar's lawyers, after Monday's brief hearing.

The tribunal has been working slowly and has generated little news because it has opted to gather written evidence rather than hear witnesses in open court.

However, uncertainty over when it will issue a ruling and what it will decide is affecting Nigerian political and economic life as government and investors fret about what will happen if Yar'Adua loses his job.

Other election tribunals, at state level, have annulled the poll victories of five state governors from the ruling party on the grounds of fraud. The state elections took place a week before the presidential vote.

Yar'Adua, who has made adherence to the rule of law his mantra since taking office on May 29, has allowed the state tribunals to work freely and pledged to respect the decision of the presidential tribunal.