The High Court found Sheik Raed Salah was wrongfully arrested at a London hotel three days after his arrival in June, The Guardian reported. Salah, a leader of the Islamic Movement in Israel, was able to get into the country because an immigration officer at Heathrow International Airport did not notice he was on an exclusion list.
After his detention, Home Secretary Theresa May ordered his deportation. Salah, who was freed on bail in July, is fighting that order.
The case was the first after May had promised to keep "non-violent extremists" out of Britain, describing them as people who do not engage directly in terrorist actions but encourage those who do.
The court found that only two days of Salah's detention was unlawful, the period before he was given the correct reason, so any compensation he receives will not be large, the newspaper said.