Joe Robinson, 25, was arrested on holiday in Turkey and accused of fighting Isis alongside the People's Protection Units of Syrian Kurdistan (YPG) - which the Turkish state considers a terrorist organisation.
The YPG is not banned in the UK and Robinson, from Accrington, Lancashire, said he only spent a month with them in Syria while providing medical support to civilians.
Ibrahim Dogus, the Founder of the Centre for Turkey Studies, is now calling for the Foreign Secretary to urgently intervene and secure Mr Robinson’s release.
Mr Dogus said: “Joe Robinson is a hero and deserves a medal not a prison sentence.
“It is unacceptable and utterly wrong for Turkey to detain and jail anyone fighting in support of the Kurdish people in Syria, who have been strong allies of the UK and the international coalition in their battle against the cult of Isis.
“The Foreign Secretary needs to step in and ensure Turkey releases Joe at the earliest opportunity, and provide all necessary support to his family.”
The Foreign Office said it is "providing consular assistance", while it is understood his case has been raised with Turkish authorities and officials will continue to do so.