Joe Robinson, from Accrington, was detained while on holiday with his mother and girlfriend last month.
The ex-soldier was charged with being a member of a terrorist organisation, after he fought alongside the People’s Protection Units of Syrian Kurdistan (YPG) against ISIS.
Yet Turkey classify the YPG as a terror group due to their links with the Kurdistan Workers’ Party, which is fighting against Erdogan’s forces in a battle for independence.
Mr Robinson is now being holed up in a cell for 23 hours a day, his solicitor told the Guardian.
Mr Robinson’s girlfriend, Mira Rojkan, has also been charged for sharing pro-Kurdish propaganda online, the solicitor added.
Theresa May has been begged to interfere in the case by Mr Robinson’s family.
So far, an online petition calling for Mr Robinson’s release has gather more than 3,000 signatures since being set up at the end of July.
The Kurdistan Solidarity Campaign said following Mr Robinson’s arrest: “Unfortunately, Joe has gone on holiday to Turkey not realising the kind of state it has now become.
“Turkey unfortunately continues to criminalise the Kurdish question and has jailed tens of thousands of Kurdish people, including elected MPs and mayors simply for peacefully campaigning for Kurdish rights.”
A Foreign Office spokesman said: “We are aware of the detention of a British national in Turkey and have requested consular access. We are providing support to the family and remain in contact with Turkish authorities.”