India, Sept. 18 -- An unprecedented revolt at Tata Sons has plunged India's largest conglomerate into a critical governance crisis, after directors voted to grant chairman N Chandrasekaran a fresh five-year extension in the face of objections by Noel Tata, who chairs the charitable trusts holding a majority share of the company. Tata termed the decision a "legal nullity" and vowed to press on with the search for a successor, opening up a chasm between the holding company and its majority-owner that lawyers expect to play out in courts.

After a meeting that ran close to four hours, the six-member board reappointed Chandrasekaran for five more years when his term ends on 20 February. Four directors voted in favour, while Tata voted against...