Nigeria's Supreme Court on Thursday anulled the election of the governor of oil-rich Bayelsa state, just hours before his inauguration and ordered his main challenger to take over.
The decision was a blow for President Muhammadu Buhari's All Progressives Congress (APC), whose candidate, David Lyon, had claimed victory in the opposition stronghold in November's vote.
The state is a key political battleground because of its oil and gas sector.
It had been held by the People's Democratic Party (PDP) since Nigeria's return to democratic rule in 1999.
The Abuja-based Supreme Court nullified Lyon's victory on the grounds that his deputy, Biobarakuma Degi-Eremienyo, had presented false information to the Independent National Electoral Commission (INEC).
The court ordered INEC to declare opposition PDP candidate Diri Duoye, who came second in the ballot, as winner. It said he should be sworn in on Friday.