Six directors at India's Reliance Energy have resigned, amid a growing family battle over ownership of the country's largest private company.
Mukesh Ambani says he is in control of Reliance
No reason was given for the departures, which were announced in a statement.
The Ambani family, which controls 47% of Reliance Industries, has denied any split between brothers Mukesh and Anil.
Shares in the firm, which has an annual turnover of 800bn rupees ($17.7bn; £9.54bn), tumbled at the news over fears of a lengthy tussle for control.
Reliance Energy closed down 34 rupees or 5.83% at 549.10, while flagship company Reliance Industries ended down 11.45 rupees or 2.22% at 503.55.
Mukesh Ambani, 47, is chairman of Reliance Industries. Anil, 45, is the chairman of Reliance Energy, which supplies energy to New Delhi and Mumbai (formerly Bombay).
Ashok Jhunjhunwala, the representative of leading shareholder Reliance Industries on the Energy board, was among those who resigned, fuelling fears that the dispute was headed for a showdown.
Shortly before the announcement, director ML Bhakta admitted to Reuters news agency that there were differences between the Ambanis.
Long-running rumours of a quarrel between the two brothers at the head of the company came to a head last week when Mukesh said there were "ownership issues" with his younger brother.
Reliance dominates India's oil industry
He later said his comments had been taken out of context.
Their father Dhirubhai Ambani is thought to have died without a will, although this has not been confirmed by the family.
Reports in the Indian press say a planned family meeting on the issue on Monday was cancelled.
On Monday, Mukesh Ambani insisted that his father had settled all ownership issues before his death.
He later sent an e-mail to Reliance employees, asserting that there was no ambiguity from his father's legacy and that he was the final authority on all matters concerning Reliance.
Reliance companies span the oil, petrochemical and telecoms sectors.
It operates India's largest petroleum refinery and produces 12 million tons a year of petrochemicals.
It also owns 45% of Reliance Infocomm, India's biggest cellular phone company, and half of Reliance Energy. The Ambani family owns 47% of Reliance group.