The San Francisco-based conservation group, WildAid, said it will submit the study conducted by a leading Thai laboratory to a Bangkok civil court as part of its defense in the lawsuit.
"We told the truth when we said that sharks are in trouble and that the people eating them may be too," WildAid Thailand president Kraisak Choonhavan told reporters.
He said the study by Chula Unisearch, a testing laboratory of Chulalongkorn University, showed that 15 of the 45 randomly purchased shark fins in Bangkok had more mercury than is considered safe for humans by the Thai Food and Drug Administration.
One sample had a mercury concentration seven times the FDA limit, according to the study, which was partially funded by WildAid.
WildAid also said shark populations around the world are in rapid decline due to overfishing and increased demand for shark fin soup.
WildAid first made the allegation about mercury levels a year ago, prompting shark fin soup restaurant owners and other retailers to sue the group for $2.6 million.
The plaintiffs said WildAid's claim had damaged their businesses. They said they will withdraw the lawsuit if WildAid apologized and withdrew its claim.
According to WildAid, sharks are predators at the top of the food chain and so end up as receptacles for all the toxic material that has been consumed by smaller fish feeding in polluted coastal waters.
Shark fin soup is highly popular among Chinese populations and is a top draw at restaurants in Bangkok's Chinatown. A bowl of high-quality shark fin soup can cost as much as $100