Meredith Alexander quit the Commission for a Sustainable London 2012 on Wednesday, saying she did not want to be part of a body that "became an apologist" for Dow Chemicals, the US firm linked to India's 1984 Bhopal gas tragedy.
Dow bought the Bhopal plant owner Union Carbide in 1999.
Alexander said a number of other panel members were also "deeply disturbed" by the company's sponsorship of a temporary decorative wrap around London's Olympic Stadium.
Her resignation prompted IOA chief Vijay Kumar Malhotra to send a second letter to International Olympic Committee (IOC) president Jacques Rogge exactly six months before the Games, saying there was no need to carry "this toxic legacy.”
.”..the resignation of Ms Meredith Alexander from the Games Ethics Committee — the Commission for Sustainable London 2012 — has vindicated IOA's stand of opposing Dow's sponsorship," Malhotra wrote in his letter, copies of which were distributed to Indian media.
"I am sure that you are well aware of the growing opposition to this sponsorship the world over with NGOs (non-governmental organizations), intellectuals like Noam Chomsky, Members of British Parliament and civil society openly coming out against it.
"On behalf of the IOA I again urge you to take steps to remove Dow as sponsor and settle the matter as early as possible," Malhotra added.
Activists say 25,000 people died in the years that followed the gas leak at a pesticides factory in the central Indian city of Bhopal in Madhya Pradesh.
Campaigners have demanded Dow boosts a 1989 compensation package for those affected by the disaster.
Dow, also an IOC worldwide partner, has denied any responsibility for the accident and says Union Carbide had settled its liabilities with the Indian government.
A number of former Olympians have slammed the London sponsorship deal while Madhya Pradesh chief minister Shivraj Singh Chauhan has urged the government to boycott the Games over the issue.
However, Malhotra has ruled out the possibility.
Malhotra said he has sent a copy of his letter to London Games chief Sebastian Coe as well, while also conveying the IOA's position to the British High Commissioner in India last week.
