HARARE, Zimbabwe — President Robert Mugabe of Zimbabwe, the world’s oldest head of state, abruptly dismissed the country’s vice president on Monday, a move that positions Mr. Mugabe’s wife, Grace, to succeed him as president.

The vice president, Emmerson Mnangagwa, who retains strong support from the military, had been seen as a main rival to Mrs. Mugabe in the internal fight over who might succeed the president, who led his country to independence in 1980 and is the only leader most Zimbabweans have ever known.

“The vice president has consistently and persistently exhibited traits of disloyalty, disrespect, deceitfulness and unreliability,” Information Minister Simon Khaya Moyo said in explaining Mr. Mugabe’s decision. “It had become evident that his conduct in his discharge of his duties had become inconsistent with his official responsibilities.”

Mr. Mnangagwa’s critics had accused him of plotting to take charge of key state institutions and of forming a set of parallel institutions within the ruling party, known as ZANU-PF.