Nigerian Vice-President Atiku Abubakar has been expelled from the ruling party for standing as an opposition candidate in next April's presidential election.
Atiku Abubakar is standing as an opposition presidential candidate
The People's Democratic Party (PDP) urged President Olusegun Obasanjo to replace his deputy with a party member.
But Mr Abubakar's spokesman says constitutionally the vice-president is allowed to remain in office.
Mr Abubakar fell out with Mr Obasanjo over the president's failed bid to seek an unconstitutional third term.
The vice-president was suspended from the PDP in September after Nigeria's anti-corruption body accused him of fraudulently using $125m (£64m) of public funds for personal business interests.
He denies the allegations, saying they are politically motivated.
The PDP's National Executive Committee claims the president is legally obliged to replace Mr Abubakar.
"Article 142 of the constitution... contains the requirement that whoever is to occupy the office of the vice-president shall be of the same political party as the president," the party statement said, Reuters news agency reports.
But Mr Abubakar's spokesman, Garba Shehu, dismissed such claims.
"The constitution allows three means by which the vice-president can leave office: impeachment, death or voluntary resignation," he told the BBC's Focus on Africa programme.
"He and the president won an election on a joint ticket. He's not a squatter on that ticket.
"Only the Nigerian people can recall him."
Mr Abubakar was chosen by the opposition Action Congress (AC) as its candidate on Wednesday.
Mr Shehu says Mr Abubakar, who is out of the country on holiday, officially resigned from the PDP on Monday - before his AC nomination.
Meanwhile, the nomination of presidential candidates has now closed with former military ruler, Ibrahim Badamasi Babangida out of the race.
Gen Babangida withdrew from the PDP primaries
Last week Mr Babangida withdrew from the ruling PDP presidential primary after it became clear that President Obasanjo was backing Umaru Yar'Adua, a northerner like Mr Abubakar.
Many analysts had expected Mr Babangida, nicknamed Maradona by the Nigerian media for his unpredictable political dribbles, to seek the nomination of another party.
But when the deadline for submission of nominees to the country's electoral body closed on Friday, Mr Babangida had still not managed to find another party.
"Submission of names of party nominees for the presidency and the Senate closed today as provided for in the Electoral Act," Ndidi Okafor of the Independent National Electoral Commission told the BBC.
There are some 50 presidential contenders but it is becoming increasingly likely that Nigerians will be choosing from among Mr Yar'Adua, Mr Abubakar or the opposition All Nigeria Peoples Party's Muhammadu Buhari come April.
Next year's polls should become the first transfer of power from one elected leader to another since independence in 1960.