A man arrested for alleged involvement in a siege carried out by radical Islamists in Bangladesh should be released immediately, his lawyer said.
Hasnat Karim, 47 - a dual British-Bangladeshi citizen - was celebrating his daughter's 13th birthday with his family when the restaurant they were in became the target of a terrorist attack.
Twenty people, mostly foreigners, were killed when gunmen attacked the Holey Artisan Bakery in Dhaka on July 1.
Security forces stormed the restaurant on July 2, killing the gunmen and rescuing the remaining 13 hostages.
Mr Karim had not been heard from since the attack, and his family say he is innocent and is being held without charge.
Rodney Dixon QC, the family lawyer, said: "The authorities have finally admitted that Mr Karim is in their custody. They have had more than sufficient time to make any inquiries.
"There is clearly no evidence to charge him and he should be let go without any further delay.
"He has been arrested under article 54 of the CPC that allows remand without charge. He is not a suspect and has not been charged. We therefore fully expect that he will be promptly released."
Mr Karim was previously a part-time lecturer at North South University in Dhaka and is now a businessman, according to Amnesty International.
University of Toronto student Tahmid Hasib Khan was arrested in a different area of Dhaka over similar allegations.
An Amnesty International spokesman said: "While it is a positive step that Hasnat Karim and Tahmid Khan have been produced in court and formally arrested, the Bangladeshi authorities must now ensure that their rights in detention are being met, including access to a lawyer of their choosing, their families and any medical attention they may require.
"We call on the Bangladeshi authorities to charge them promptly with a recognisable crime, in line with international law and standards, or else release them.
"There must be justice for the victims of the horrific attack in Dhaka, but Bangladesh must ensure that human rights are now not sacrificed in the name of national security."
A Foreign Office spokesman said: "We are in contact with local authorities following the arrest of a Bangladeshi-British citizen in Dhaka on August 3."