Nigeria's Vice-President Atiku Abubakar has been omitted from the electoral commission's list of 24 candidates for next month's presidential election.
Many see Atiku Abubakar as the ruling party's main challenger
Electoral officials say corruption charges against him would have to be dropped for him to be eligible to run for the presidency.
Mr Abubakar became a leading opposition candidate after falling out with former ally President Olusegun Obasanjo.
He has said he will challenge the decision in court.
Despite a high court ruling that the commission could not disqualify candidates, electoral officials say the constitution bars Mr Abubakar from running for office.
"Commissioners have no choice but to apply the constitution," Independent National Electoral Commission (Inec) Chairman Maurice Iwu said.
"Once you have been indicted, Inec cannot do anything about it."
Time running out
Mr Abubakar has said the corruption indictment is politically motivated and he has challenged it in the courts.
The BBC's Alex Last in Nigeria says the whole issue of whether he can stand will be resolved in the courts, but there is one rather large problem: time is running out.
There are just five weeks to go before the election; ballot papers will have to be printed and delivered soon, given that there are some 60 million registered voters spread throughout the country.
There is the real risk that these elections will be mired in messy court cases and legal challenges, both before and after the votes are cast, our reporter says.
The vice-president's party, the Action Congress, says the position of Inec is leading Nigeria towards anarchy.
"I am confident that our candidate will contest the elections because Inec has no powers under the constitution or the Electoral Act to disqualify any candidate," said party spokesman Lai Mohammed.
"We are already in the courts over this matter and I am sure the courts will compel Inec to put him on the ballot."
The Action Congress is one of the two main opposition parties contesting the election, and continues to hold mass rallies around the country, despite the block to its candidates' presidential ambitions.
President Obasanjo is standing down after two terms and Umaru Yar'Adua is standing on behalf of his People's Democratic Party (PDP).
Former military ruler Muhammadu Buhari of the All Nigeria People's Party (ANPP) is the best known challenger on the list of 24 approved candidates.