"I firmly declare that in protest against the illegal and inhumane behaviour of the judicial and security apparatus and their hostage-taking, I have started a hunger strike since the morning of the 12th of Bahman (February 1)," wrote Panahi.
"I will refuse to eat and drink any food and medicine until the time of my release. I will remain in this state until perhaps my lifeless body is freed from prison."
The director was arrested in early July amid a crackdown on freedom of expression, after going to Evin prison to enquire the about whereabouts of filmmakers Mohammad Rasoulof and Mostafa Al-Ahmad following their detention a few days previously.
It was announced a few days later that the Iranian authorities had decided to reactivate a six-year sentence originally meted out to Panahi in 2010 alongside a 20-year filmmaking and travel ban.
The charges and sentence were connected to his attendance at the funeral in 2009 of a student shot dead in the Green Revolution and his later attempt to shoot a feature set against the backdrop of the uprising.
The director has not left Iran since the sentence and has been under house arrest on and off during this period.
Nikbakht argued successfully at the Supreme Court in October that the six-year sentence had passed Iran's 10-year statute of limitations period and was no longer applicable.
He was granted permission to apply for a retrial in a move that should have resulted in Panahi automatically being released on bail, but Iranian authorities have hindered the process of activating this until now.
"While we have seen that it takes less than 30 days from the time of arrest to the hanging of the innocent youth of our country, it took more than a hundred days to transfer my case to the branch with the intervention of security forces," wrote Panahi.
The director said Iranian authorities had made repeated excuses as to why he was not being released, reports 'Deadline'.
"What is certain is that the violent and illegal behaviour of the security institution and the reckless surrender of the judiciary once again shows the implementation of selective and tasteful laws," he wrote.
"It is only an excuse for repression. I knew that the judicial system and the security institutions have no will to implement the law (which they brag about), but out of respect for my lawyers and friends, I went through all the legal channels to fight for my rights," he continued.
"Today, like many people trapped in Iran, I have no choice but to protest against these inhumane behaviours with my dearest possession, that is, my life."
Panahi's detention preceded Iran's 'Women, Life, Freedom' uprising sparked by the killing of Mahsa Amini while in police custody September 16 for not wearing her hijab in accordance with Iran's religion-based law.