Turkey's Zorlu Holding said late on Saturday its chief executive Cem Koksal had resigned, after authorities launched a probe into an email exchange for allegedly limiting freedom of belief over the celebration of the Muslim holy month of Ramadan.

On Friday, Ergun Guler, chief executive of electronics firm Vestel, which is a subsidiary of Zorlu Holding, sent a message to employees celebrating the start of Ramadan, which began on Saturday.

Koksal later sent an internal email saying the company only celebrated Eid al-Fitr and Eid al-Adha officially, not Ramadan, and that it wanted to maintain a secular stance as it worked to become a multinational corporation, with employees from various beliefs and origins.

After images of that email were circulated by local media and on social media, an Istanbul prosecutor's office launched an investigation into the issue, the state-owned Anadolu news agency said, adding that Koksal was detained on Saturday for "obstructing the exercise of freedom of belief, thought, and expression".

In a statement, Zorlu Holding later said it ackowledged the "sensitivity" caused by the "internal argument over management principles", and that Koksal had resigned as of March 1.

"It is known to the public that Zorlu Holding has been an institution with family values -- and business ethics since 1953, and has been integrated with the national and spiritual values -- of our country," it said.

"As the Zorlu Group, we convey our sadness due to what has happened to all our stakeholders and the public," it added.

The conglomerate operates in the energy, real estate, electronics, white goods and textile sectors.

The investigation into Koksal comes after a battle between President Tayyip Erdogan and Turkey's biggest business group TUSIAD last month over comments from executives in the group criticising a government crackdown on opposition figures.

(Reuters)

Cem Koksal

Turkey's Zorlu says CEO resigns after probe into Ramadan email