Construction of a new structure in place of the Mullaperiyar dam will be a grave threat to Tamil Nadu as it will deny water to a vast stretch of cultivable land in the southern districts, Marumalarchi Dravida Munnetra Kazhagam general secretary Vaiko said here on Wednesday. Addressing party cadre at a demonstration held near the Collectorate, he condemned the Centre for granting permission to the Kerala government for undertaking a survey, which, he claimed, was a prelude to the construction of a new dam. Accusing the State government of failing to act at the appropriate time, he said the Centre was part of a conspiracy against the interests of Tamils in the issue. Mr. Vaiko expressed concern over the frequent attacks on Tamil Nadu fishermen allegedly by the Sri Lankan navy.