Zimbabwe vice-president Emmerson Mnangagwa said late on Thursday he had been hospitalised in South Africa in August because he had been poisoned, escalating confrontation in the country during a fight to succeed 93-year-old President Robert Mugabe.

Mnangagwa, a former intelligence chief nicknamed “the Crocodile”, is the leading candidate to succeed Mugabe, the only leader Zimbabwe has known since independence in 1980.

He did not say who he believed was responsible for trying to kill him, but his main political rival, First Lady Grace Mugabe, swiftly denied having anything to do with it.

Mnangagwa was airlifted to Johannesburg after falling ill in August. At a news conference late on Thursday open only to state media, he said doctors had concluded that poisoning was to blame for his illness, and not inadvertent food poisoning.

“The medical doctors who attended to me ruled out food poisoning but confirmed that indeed poisoning had occurred and investigations were in progress,” Mnangagwa said, reading from a statement. He provided no further details or proof.

Mnangagwa, 75, became vice-president in 2014, putting him at the front of the pack to succeed Mugabe. However over the last 18 months he has met fierce opposition from Grace Mugabe and a faction of the ruling party backing her.

The first lady denied having anything to do with his illness and accused him of lying about it to get public sympathy.

“Why should I kill Mnangagwa? Who is Mnangagwa on this earth?” Grace Mugabe said in footage aired on Friday on state television. “Killing someone who was given a job by my husband? That is nonsensical.”

After his hospitalisation, Zimbabwean media said Mnangagwa had suffered food poisoning after eating ice cream from a dairy company owned by Mugabe and his wife, which the Mugabes both denied.