Radical Muslim cleric Abu Hamza has won his appeal against government attempts to strip him of his British passport.
A Special Immigration Appeals Commission (Siac) upheld his appeal in a 12-page ruling.
The preacher had said he would be left "stateless" as he had already lost his Egyptian citizenship.
A Downing Street spokeswoman said David Cameron was "disappointed" but it would not affect ongoing extradition proceedings.
Abu Hamza, 52, was jailed for seven years in February 2006 for inciting murder and race hate.
At a three-day hearing in London last month, Abu Hamza's lawyers argued he had already been stripped of his Egyptian citizenship so could not have his British passport taken too, as that would render him "stateless".
But the Home Office said there was no documentation to prove he was no longer an Egyptian national and though he was once denied an Egyptian passport, he was later allowed one.
Mr Justice Mitting ruled it was unclear whether Abu Hamza was stripped of his Egyptian nationality before or after the then home secretary David Blunkett gave notice of his intention to strip the cleric of his British citizenship on 4 April, 2003.
But he said the panel heard from experts who "had very good grounds for believing, and did believe, that a decree had been issued, probably unpublished, which effectively stripped the appellant (Hamza) of his (Egyptian) nationality".
Mr Justice Mitting went on: "All that we can be satisfied about, on balance of probabilities, is that a decree has been issued and that its effect is to deprive the appellant of Egyptian nationality.
"It is immaterial that the decree was almost certainly issued after the then secretary of state gave notice of his intention to deprive the appellant of his British citizenship on April 4 2003.
"Because the secretary of state cannot make a deprivation order until his appeal has been determined, Siac must take into account all relevant facts and circumstances, whether they occurred before or after notice was given."
Abu Hamza is in Belmarsh Prison as he challenges attempts to extradite him to the US.
A Home Office spokesperson said: 'We are extremely disappointed by today's judgement and will be considering it closely.
"British nationality is a privilege and the home secretary has the ability to remove it from dual nationals when she believes it to be in the public good."