Shares of HDFC rallied as much as 16.5 per cent on the BSE, the most in nearly two decades, data from BSE showed after its board approved the merger with the country's largest private lender HDFC Bank. Meanwhile, HDFC Bank rallied the most since March 25, 2020. Shares in HDFC Bank jumped as much as 14.3 per cent to hit an intraday high of Rs 1,721.85.