El Al flies to Mumbai four times a week, along a route over Ethiopia and then east to India that, by avoiding Saudi airspace, has a journey time of seven hours rather than five.
"Air India will gain a significant and unfair advantage over El Al with respect to flights between Israel and India", CEO Gonen Usishkin said in the letter.
Tickets for the new route went on sale shortly after the announcement Wednesday, reported the Times of Israel. This route, too, is covered in 7 hours and 10 minutes.
Israel's national carrier El Al now operates an India service that takes a detour over the Red Sea to avoid flying over Saudi Arabia and Iran.
Air India received a grant of $925,000 (NIS 3.2 million) from the Israeli Ministry of Tourism in order to operate three weekly flights on the new route, the ministry told Ynet News on Wednesday. On its return leg, the plane will depart from Tel Aviv at 10.15 pm and land in Delhi at 9 am the day after, according to a statement from Air India.