Mahdi Hashi, 23, of Camden, was stripped of his citizenship in the summer when he went missing in Africa.
He appeared in a New York court on Friday charged with “providing material support” to terrorist group Al Shabaab.
Solicitor Saghir Hussain said the case has “hallmarks of rendition”.
The Home Office has refused to comment on the issue saying it was a matter of national security.
Mr Hashi was born in Somalia and moved to London with his family at the age of five. Three years ago he returned to Somalia and got married.
‘Training with Al Shabaab’
His family got a letter from the Home Office earlier this summer saying his citizenship was being revoked due to his “extremist” activities.
Soon after Mr Hashi went missing.
The family learnt of him being in New York after he appeared before a District Court.
A press release from the Federal Bureau of Investigation (FBI) said Mr Hashi, also referred to as “Tahla”, and two other defendants, were arrested in August by local authorities in Africa on their way to Yemen.
In October, “a grand jury in the Eastern District of New York returned a sealed indictment against the defendants” and on 14 November the FBI “took custody of the defendants and brought them to the Eastern District of New York”, it added.
On 15 November the defendants were charged with conspiracy to provide material support to Al Shabaab, unlawful use of machine guns, and it is alleged that between December 2008 and August 2012, the men “participated in weapons and explosives training with members and associates of Al Shabaab”.
If convicted of all charges, each defendant faces a mandatory minimum sentence of 30 years and a maximum sentence of life in prison.
Mr Hussain, who acts for Mr Hashi’s family, said: “This has all the hallmarks of a rendition whereby somebody is picked up secretly and transferred into secret detention and thereafter transferred to another jurisdiction and here it’s the Americans.
“Here is a young lad from London, has lived in London most of his life, has travelled to Somalia to meet his family and then he is picked up.
“Coincidentally his British citizenship is withdrawn around the same time that he is kidnapped and this is where we will be asking the government questions as to what knowledge do they have, did they conspire with the Americans to deprive him of his citizenship so that to enable them to kidnap him and hold him in secret detention.”
Mr Hussain added that if the UK Government had not revoked the man’s citizenship then “it would have caused severe difficulties for British and American relations for a British citizen to be picked up from somewhere in the world and transferred to America without any due process whatsoever”.