Karunanidhi told reporters that he had already written to Prime Minister Manmohan Singh regarding the matter.
"A case is pending in the Supreme Court and we will take legal steps to get our due share of water," he said, when asked about Karnataka turning down Tamil Nadu's request.
Karnataka had yesterday turned down Tamil Nadu's plea for releasing water to Mettur Dam "in the present circumstances" but said the excess inflow would anyway reach there.
Karnataka is required to release 205 TMC of water annually to Tamil Nadu as per the interim award of inter-state water dispute tribunal.
Cauvery river is the main irrigation source for farmers in delta districts of Tamil Nadu.
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