Top Nigerian Muslim cleric Sheikh Ibrahim Zakzaky, who has been imprisoned for four years now, is said to have been poisoned in prison, according to doctors who found high levels of toxic substance in his blood.

Members of Islamic Movement in Nigeria (IMN) announced that their cleric was poisoned, citing results of an investigation by the movement's Academic Forum.

Based on the findings, a high-level of toxicity has been diagnosed in the cleric’s blood, which has worsened his health condition.

Zakzaky’s legal team has long called for his release, saying he is suffering from health issues that require urgent medical care abroad, but the state high court in Kaduna has denied the request.

A team of medics examining Zakzaky in prison have renewed pleas for him to be released, saying levels of the chemical toxic substance — lead — is so dangerously high in his blood that requires an immediate medical treatment.

A medical team sent by International Healthcare Research Center (IHRC) to Nigeria in April said that the imprisoned cleric was in urgent need of medical care that could only be fully accessed outside Nigeria.

Sheikh Zakzaky, who is in his mid-sixties, lost his left eyesight in a 2015 raid by security forces, that left more than 300 of his followers and three of his sons dead. His wife also sustained serious wounds.

He has been kept in custody along with his wife and a large number of his followers ever since.

Back in 2016, Nigeria’s federal high court ordered his unconditional release from jail following a trial, but the government has so far refused to set him free.