Mr Abubakar, the candidate of the Peoples Democratic Party (PDP), announced the choice of Mr Okowa, a state governor, on Thursday.

Political parties in Nigeria have until Friday to submit the names of their presidential candidates and running mates.

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Mr Okowa, a former senator is into a second four-year term as governor of Delta state, which is in southern Nigeria.

Mr Abubakar, a Muslim from northern Nigeria was expected to pick a running mate from the Christian-majority south of the country so his choice has come as no surprise.

However, many are worried about Mr Okowa's links to James Ibori, a former governor of Delta state convicted of money laundering in the UK in 2012.

Ibori is still highly influential in Delta state politics and, many believe, had a hand in the election of Mr Okowa as governor.

The PDP is seeking to reclaim power from the ruling All Progressives Congress (APC), which will field Bola Tinubu as its presidential candidate.

Mr Tinubu is yet to announce a running mate.