Yar'Adua's two main rivals in last April's election, former military ruler Muhammadu Buhari and then Vice-President Atiku Abubakar, have asked the tribunal to annul the result, alleging widespread fraud. EU observers judged the polls "not credible".

Buhari's lawyer Mike Ahamba said on Thursday the tribunal had informed him it would deliver the ruling starting at 10 a.m. (0900 GMT) on Tuesday.

Yar'Adua's victory was disputed from the moment it was announced and doubts over whether he would finish his four-year term have delayed policy-making and investment decisions in the world's eighth-biggest exporter of crude oil.

The tribunal heard final submissions on Feb. 5 but had not previously said when it would deliver its ruling.

It would be unprecedented for an electoral tribunal to nullify a presidential election result in Nigeria.

Yar'Adua, who has made respect for the rule of law his watchword, has pledged to respect the tribunal's decision. He has not said whether he would appeal or whether he would stand again in the event of a re-run.

He was on a visit to remote northeastern Borno state on Thursday. He was scheduled to travel to China for an official visit next week and there was no immediate comment from his office on whether he would go ahead or cancel the trip.

UNCERTAINTY

The constitution and the Electoral Act do not spell out exactly what would happen after an annulment, giving the special tribunal some discretion in interpreting the laws.

Based on what the texts say about other possible scenarios in which the president would lose his job, experts say Yar'Adua would likely have 21 days to appeal to the Supreme Court, during which time he would hold onto his position.

If Yar'Adua loses the appeal or decides against an appeal, he would probably stand down and the Senate president would take over for 90 days during which a re-run would be held.

This option might be problematic, however, because Senate President David Mark's own election as a senator is being disputed at a tribunal in his home state of Benue.

Should the tribunal uphold the result, Buhari and Abubakar would be likely to appeal against the ruling.

Official results from the election on April 21 last year gave Yar'Adua 24.6 million votes, compared with 6.6 million for Buhari and 2.6 million for Abubakar.

European Union monitors said the elections "fell far short of basic international standards" because of widespread vote-rigging and intimidation, mostly by the ruling People's Democratic Party (PDP).

A week before the presidential poll, Nigeria held elections for state governors and those were equally contentious. Six state governors from the PDP have since seen their poll victories annulled by courts because of irregularities.

That has aroused speculation that Yar'Adua could be next, although many political insiders believe this unlikely because of the risk of instability in a country with a long history of botched elections and military coups.