The expelled chairman of Tata group, Cyrus Mistry, has taken his boardroom battle public by releasing his lengthy, aggressive email as the $100bn business empire reels under the aftermath of the ouster. Mistry's letter to the directors of the group holding company, Tata Sons, makes several allegations against his predecessor, Ratan Tata, who has now taken over as interim chairman, and the conglomerate's board of directors.

Following the disclosure of the email, India's two main trading exchanges sought an explanation from the 148-year-old group. Stocks of Tata group companies continue to plunge losing Rs 550bn (£6.72bn) in market cap on the third day since Mistry's unceremonious exit. The group's coveted cash cow, Tata Consultancy Services, was not immune to the latest hit.

In another rare step, the country's market regulator Sebi (Securities and Exchange Board of India) has said they are closely monitoring the developments at the group.

While the government asked politicians to keep away from the spat, which is turning into the most spectacular corporate battle in the recent past in India, lawyers from both sides are bracing for a legal battle.

Business fights of this magnitude are not unheard of in India. The Ambani brothers were engaged in a bitter battle, but the fact that it is happening in the Tatas, one of India's most revered conglomerates, has stunned all.

"It has taken everyone by surprise. Nobody would have thought such things could happen at Tata," J N Gupta, a former executive at a regulator and currently the chief of Stakeholders Empowerment Services, told Reuters.

The 48-year-old ousted chairman's parting shot surfaced in the public domain late on Wednesday (26 October) prompting India's leading newspapers to run dramatic headlines. The five-page letter called some of the struggling companies such as Tata Steel Europe and Tata Motors as "legacy hotspots" in what is seen as a veiled attack on Ratan.

Ratan's pet small car Nano which was touted as the world's cheapest car costing Rs 100,000, has also drawn flak from Mistry. The 78-year-old patriarch, Mistry wrote: "As there is no line of sight to profitability for the Nano, any turnaround strategy for the company requires to shut it down. Emotional reasons alone have kept us away from this crucial decision."

Tata Sons has not yet responded to Mistry's letter.