Cyrus Mistry, the ousted chairman of Tata Sons, has resigned from all Tata Group companies at the start of a week when four of the firms were to vote on removing him as director in extraordinary general meetings or EGMs. "It is time to shift gears, up the momentum and be more incisive in securing the best interests of the Tata group," Mr Mistry said in a letter to the Board of Tata Sons and shareholders today.