Nigeria's ruling party has suspended the vice-president from its ranks for three months over corruption reports.
Atiku Abubakar helped thwart a third term for President Obasanjo
Atiku Abubakar denied the claims, saying they were politically motivated.
He hopes to contest next year's general election and backed a campaign to prevent President Olusegun Obasanjo from seeking a third term.
Mr Abubakar will remain vice-president but his suspension means he will not be eligible to seek the ruling PDP's presidential nomination.
He condemned the decision of the People's Democratic Party (PDP) as an "evil plot" to keep him out of the 2007 race.
"By the grace of God, this sinister plot will fail," he said in a statement.
The suspension is the latest blow to Mr Abubakar's presidential ambitions, the BBC's Alex Last reports from Lagos.
He is already feuding with the president over corruption allegations which could bar him from contesting next year's elections, our correspondent notes.
The PDP is the largest party in Nigeria, has the most money and, more importantly, is in power, so it is viewed as the best place to be if running for office, says our correspondent.