Mr Abubakar had "rediscovered and repositioned himself" and would make a better president than Mr Buhari, Mr Obasanjo said, after holding talks with him on Thursday.

The two had a strained relationship, even though Mr Abubakar was Mr Obasanjo's deputy in government from 1999 to 2007.

He led the charge against Mr Obasanjo's bid to change the constitution to run for a third term, and Mr Obasanjo was forced to step down in 2007.

Mr Abubakar broke away from the-then ruling People's Democratic Party (PDP), forming a rival party that failed to propel him to the presidency.

Last week, the PDP chose him as its candidate in next year's election, with Mr Obasanjo saying that the "prodigal son" had returned.

Mr Buhari's campaign team dismissed Mr Obasango's endorsement of Mr Abubakar as a “complete non-event”.

"They will lose together," Mr Buhari's spokesman Garba Shehu said.