Nnamdi Kanu’s family in London has accused British government of failing to provide him with consular assistance in Nigeria’s DSS detention.
Kanu holds British and Nigerian citizenship.
Lawyers for the family in UK made the claim of abandonment on Thursday.
Kanu, leader of the Indigenous People of Biafra (IPOB) group that campaigns for part of southeastern Nigeria to secede, was kidnapped back to Nigeria in June after years on the run abroad.
He is being held pending a treason trial.
Bindmans, a London law firm, said it had sent a pre-action letter to British Foreign Secretary Dominic Raab on behalf of Kanu’s family, challenging Britain’s failure to assist him.
A junior Foreign Office minister had said last week that Britain stood ready to provide consular assistance to Kanu and had requested consular access from the Nigerian government.
However, Bindmans said no consular visit had taken place.
“Despite the family’s concern that he has been subject to torture and is being denied essential medical treatment, the British High Commission have so far failed to visit him in detention,” it said in a statement.
Kanu was supposed to appear in court in Abuja on Monday but the authorities failed to produce him, citing logistical issues.
His trial was adjourned until October.