Nigeria's two main opposition presidential candidates will not boycott Saturday's polls and will run individually, their parties have said.
It scuppers the idea of the opposition uniting behind one candidate.
Atiku Abubakar's spokesman told the BBC that Muhammadu Buhari's party, which took the decision not to boycott, had "crashed the alliance".
Earlier, the government rejected an opposition call for a postponement after last week's flawed state polls.
There were violent clashes and cases of vote rigging, mostly in the oil-rich south.
NIGERIAN ELECTION 21 APRIL
60m registered voters
120,000 ballot boxes
24 presidential candidates
Main contenders: Atiku Abubakar for the AC, 60 years oldMuhammadu Buhari for the ANPP, 64 years oldUmaru Musa Yar'Adua for the PDP, 55 years old
Polls open at 0800 (local time) and close at 1500
To avoid a run-off, a candidate needs the highest number of votes overall and at least 25% of votes in 24 of the 36 states
The ruling People's Democratic Party (PDP) won 27 of the 34 state contests declared. Two elections were postponed because of irregularities.
The BBC's Will Ross in the capital, Abuja, says the move will come as welcome news for the governing PDP as it will split the opposition vote.
The governing party is also well aware that had the opposition boycotted the election, the entire legitimacy of the polls would have been brought into question, he says.
Opposition politicians accuse the governing party of trying to rig the election and suggest the electoral commission is far from independent.
The PDP says the opposition is crying foul because it knows it will lose Saturday's presidential and legislative elections.
The boycott had been favoured by the Mr Abubakar of the Action Congress (AC) party.
"We shall not boycott. I repeat, we shall not boycott and we will participate fully in the National Assembly elections and the presidential elections," said All Nigeria People's Party chairman Edwin Ume-Ezeoke, who is also Mr Buhari's running mate.
"The party has decided that as true and patriotic democrats, we do not want to unconsciously become tools in the hands of the PDP-led government in their tenure elongation programme."
Mr Abubakar agreed with these sentiments.
"You have to participate to challenge the wrongdoings in the courts," he told the BBC's Focus on Africa programme.
He said it would be better to field a common opposition candidate, but Mr Buhari disagreed that the failure to do so would hand over victory to the PDP.
"It hasn't handed it over. If that is your assumption, it is the wrong assumption," he told the BBC.
Mr Abubakar's participation in the poll was only confirmed on Tuesday by the election commission, after a Supreme Court ruling on Monday.
The Independent National Election Commission (Inec) had disqualified him because of charges of corruption, which he denies.
Many Nigerians fear further violence on Saturday, which should lead to the first handover of power in Africa's most populous nation from one civilian administration to another.
Meanwhile, in the northern city of Kano, the military says it is continuing to clear a group of Islamic militants away from the Panshekara district on the outskirts of the city after two days of clashes, which have left at least 38 people dead.