Nigeria's former President Olusegun Obasanjo has quit the ruling People's Democratic Party (PDP) ahead of the 28 March elections, tearing up his membership card in public.
Mr Obasanjo has been fiercely critical of President Goodluck Jonathan, who is seeking re-election on a PDP ticket.
Mr Jonathan is facing a strong challenge from opposition candidate Muhammadu Buhari.
The elections, due on 14 February, were postponed over security concerns.
BBC Nigeria analyst Ibrahim Shehu Adamu says Mr Obasanjo's decision is a major blow to the PDP, showing the divisions that have hit the party as it battles to extend its 15-year rule.
'Serious embarrassment'In a statement, the PDP described Mr Obasanjo as a "revered leader of our party" and said it was "deeply saddened" by his resignation.
It added that it was "bewildered" by the former president's choice to tear up his party membership card in public.
Mr Obasanjo, 77, was a founding member of the PDP and led the party to two resounding victories after military rule ended in 1999.
So, his decision to quit the PDP may knock the morale of some party activists and persuade undecided voters to back the opposition in the hard-fought presidential and parliamentary elections.
But Mr Obasanjo's move does not come as a complete surprise. His influence within the party has been declining for quite some time and he probably found it difficult to see President Jonathan - whom he once mentored - ignoring his advice.
More worrying for Nigerians will be the role of the military. There are growing suspicions that it is backing Mr Jonathan, raising doubts about its impartiality and the credibility of the elections.
Last week, Mr Obasanjo raised fears of a coup. Many Nigerians will be hoping that his fears are misplaced and that Nigeria holds a free and fair election in which the losing candidate gracefully accepts defeat.
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