Chris Stephenson, a lecturer at Istanbul's Bilgi University, said he was put on a plane late yesterday after turning up to court to support fellow academics charged with terror offences.
They were detained after signing a petition condemning military actions in operations against the outlawed Kurdistan Workers' Party (PKK).
"At the entry (to court) they searched my bag and found some invitations for Kurdish New Year from the People's Democratic Party, which is a legal party," the 66-year-old explained to AFP.
State news agency Anatolia reported that Stephenson was questioned by police after they found him in possession of leaflets promoting Kurdish New Year celebrations.
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Britain's Foreign Office said it was providing assistance "to a British national who was arrested in Istanbul on 15 March", but did not confirm he had been deported.
The computer sciences teacher said he had become a target after signing the petition but insisted he had done nothing illegal.
"It's quite ridiculous," he said. "The invitation in my bag... It's like a postcard and was printed by the third largest party in the Turkish parliament.
Stephenson has lived in Turkey for 25 years. He married a Turkish woman 19 years ago and the couple have a Turkish daughter, aged 13.
"My life is in Turkey," he explained. "We will apply to the courts to have this decision cancelled."
The three lecturers were detained on Tuesday as the country's security services crack down on Kurdish rebels following a suicide car bomb blamed on the PKK that killed 35 people in the capital Ankara on Sunday.
Turkish President Recep Tayyip Erdogan yesterday stepped up his efforts to have pro-Kurdish lawmakers prosecuted while police detained eight pro-Kurdish lawyers in a dawn raid.