In the five-page email sent a day after he was sacked, Mr Mistry accused Ratan Tata, who is both his predecessor and his replacement, of interference that reduced him to a "lame duck Chairman."
Mr Mistry, 48, warns the board of the $103 billion group that it faces a potential drop in value of nearly $18 billion due to investments in unprofitable businesses - five areas that he tags as "legacy hotspots." The debts of the Tata Group currently run close to $30 billion.
In late 2012, Mr Mistry became the first outsider from the founding family to head India's largest conglomerate. He has been replaced by Mr Tata, 78, for four months while a permanent new chief is identified.
There was a clear mismatch of values between Mr Mistry and Mr Tata, said Harish Salve, a long-time legal adviser to the group. He told NDTV that Mr Mistry's decisions to sell assets acquired by the group's patriarch - including its steel interests in the UK - had not gone down well with the board and were seen as doing international damage to the company's formidable reputation.
Other sources who spoke on the condition of anonymity said Mr Tata believed that he would be "thrown under the bus" by Mr Mistry for the DoCoMo deal. In 2010, Japanese firm DoCoMo bought into Tatas' telecom company, but decided to exit in 2014 because it was accumulating subscribers more slowly than rivals. After Tata was unable to find a buyer, it was stopped from acquiring DoCoMo's stake by the Reserve Bank of India. Tata has now been ordered to pay $1.2 billion to DoCoMo by an international arbitration court.
Lobbing back allegations that he presided over sliding revenues, Mr Mistry details deals, including in aviation, where he says he was given no say by Mr Tata. "I cannot believe I was removed on grounds of non-performance," he said.
Mr Mistry also says that he had been guaranteed "a free hand" before he took over as Chairman with Mr Tata receding to provide "advice and guidance as when needed," but the rules were revised after his appointment to impede his powers.
As an example of Mr Tata's alleged interference, he points to the group's refusal to shut down the loss-making Tata Nano project, claiming, "Emotional reasons alone have kept us away from this crucial decision."