The UK has denied involvement in the arrest and extradition of Nnamdi Kanu, who leads a banned group seeking a breakaway state in south-eastern Nigeria, where the ethnic Igbo people form the majority.
Mr Kanu, who holds British and Nigerian citizenship, was detained on Sunday with the help of Interpol and flown back to Nigeria, according to the attorney general.
He faces treason-related charges in Nigeria, but fled the country in 2017 while on bail.
There has been speculation that he might have been arrested in the UK, where it is believed he was mainly based.
“We can confirm that Nnamdi Kanu was not arrested in the UK for extradition purposes,’’ a British High Commission spokesperson told the BBC.
He is currently in the custody of Nigeria’s secret police after a brief court appearance on Tuesday.
His trial is expected to resume on 26 July.
The 53-year-old came to prominence in 2009 when he started Radio Biafra and broadcast to Nigeria from London, using the platform to call for Biafran independence and urging his followers to take up arms against the Nigerian state.
Five years later, he founded the Indigenous People of Biafra (Ipob), which has since been labelled a terrorist organisation by the Nigerian authorities.
Mr Kanu’s initial arrest in 2015 triggered massive protests.