Atiku Abubakar: Nigeria's former VP leaves PDP for APC
Nigeria's former Vice-President Atiku Abubakar has joined the opposition - the most high-profile in a wave of defections from the governing party.
Mr Abubakar told the BBC that he believed that Nigeria should have a two-party political system.
The People's Democratic Party has won every national election since the end of military rule in 1999.
But it is deeply divided over whether President Goodluck Jonathan should seek re-election next year.
Last week, 11 senators announced that they were joining the opposition All Progressives Congress (APC) - the same party as Mr Abubakar.
The PDP has also lost its majority in the house of representatives, the lower house of parliament.
The APC has called on its members to block all legislation, including the 2014 budget.
Mr Abubakar has previously left the PDP but rejoined and sought to be the party's candidate in the 2011 elections, won by Mr Jonathan.
In an interview with the BBC's Newsday programme, Mr Abubakar denied that he was only switching sides so he could run for president.
"I really want to see a strong second party for the party," he said.
BBC Nigeria analyst Aliyu Tanko says Mr Abubakar is the most significant national politician to defect from the PDP to the APC.
Mr Abubakar is extremely wealthy and retains considerable support, especially in the mainly Muslim north of Nigeria.
Some in the PDP accuse Mr Jonathan of planning to renege on a promise to stand down after a single term as president.
The party has traditionally alternated power between northerners and those from the mainly Christian and animist south.
Mr Jonathan first became president in 2010, following the death of his predecessor, Umaru Yar'Adua.
He has not said whether he intends to seek re-election.