Palestinian activist Raed Salah has lost his deportation case and could now be forcibly removed from Britain.
An Immigration Tribunal found in favour of Home Secretary Theresa May's order that Sheikh Salah should be banned from Britain as his presence "would not be conducive to the public good".
The judgment found she had acted correctly on grounds of his alleged "unacceptable behaviour".
Sheikh Salah is expected to appeal against the decision.
A Home Office spokesman said: "We are pleased the court agrees Sheikh Salah's removal would be conducive to the public good and that he has engaged in unacceptable behaviour. We will seek to deport him at the earliest opportunity."
Sheikh Salah, the leader of the northern branch of the Islamic Movement in Israel, was detained in London in June after entering despite Mrs May imposing the banning order.
The tribunal ruled that a poem, said to be antisemitic and written by Sheikh Salah, was not racist and was not "directed at the Jewish people as a whole".
In its judgment the tribunal acknowledged that Sheikh Salah had "behaved lawfully throughout this matter, and that he has been the victim of unfairness and procedural irregularity…and was detained unlawfully for a period of time".
A Community Security Trust spokesman said: "CST welcomed the government's tightening of anti-extremist legislation, and we are glad to see it being supported by this important test case."
The 52-year-old father-of-eight won the right to seek damages earlier this month after a High Court judge ruled his detention had been partially unlawful.