By Kwasi Kpodo

ACCRA (Reuters) - Ghana's parliament speaker on Thursday dismissed an opposition motion for an inquiry into the alleged gift of a luxury car to President John Mahama, heading off the risk of an impeachment process against the West African leader.

The dispute gave Mahama's political rivals a chance to challenge his integrity in the run-up to an election in December in which he faces a stiff challenge from New Patriotic Party (NPP) leader Nana Akufo-Addo.

The NPP has alleged that Burkinabe businessman Djibril Kanazoe may have won a $613,000 government construction contract by giving Mahama a Ford Expedition sports utility vehicle - an accusation dismissed by the government.

It filed an urgent motion in parliament last month that forced Speaker Edward Adjaho to recall MPs from recess on Thursday to consider its request for an investigation.

But Adjaho said he found the request "inadmissible" because the case was already before Ghana's anti-graft agency, which is constitutionally mandated to investigate allegations of breach of conduct by public officers, including the president.

"As Speaker of this House, I am of the firm conviction that constitutional bodies must respect each other in the performance of their duties in order to avoid roles conflict," he said to cheers from government MPs and jeers from the opposition bench.

Parliamentary opposition leader Osei Kyei-Mensah Bonsu rejected the ruling by Adjaho, a member of Mahama's party, and said his side would soon announce another next course of action.

The government said in June that Mahama did not personally award the contract to Kanazoe. It also published a document it said showed that the Ford Expedition car was not given to him directly but was instead placed in a car pool at the presidency.

(Reporting by Kwasi Kpodo; editing by Aaron Ross and Mark Heinrich)