As Tamil Nadu raised the pitch for Cauvery Management Board, Karnataka Chief Minister Siddaramaiah today dismissed as ''politically motivated'' his counterpart Jayalalithaa's fresh missive to the Centre pressing her state's demand.

"Jayalalithaa is doing it for the sake of politics. This is nothing but politically motivated.

Legally it (forming the Board) cannot be done," Siddaramaiah told reporters in a sharp reaction to Jayalalithaa's letter to Prime Minister Narendra Modi.

Both Jayalalithaa and Siddaramaiah have met the Prime Minister making out their case recently with Karnataka also taking an all party delegation to Modi to resist any move for constituting the Board.

"The matter is seized of by the Supreme Court. Our all party delegation had met Prime Minister and explained him everything. Prime Minister has said such a proposal is not there. What we can do if she (Jayalalithaa) writes a letter," Siddaramaiah said.

Writing another letter to Modi today in an apparent move to pile pressure on the Centre, Jayalalithaa said "...the Constitution of the Cauvery Management Board is not optional but an integral part of the final order (of the Cauvery tribunal) itself."

Jayalalithaa's letter comes a day after Union Water Resource Minister Uma Bharti said the proposal for setting up the Board was never there and there was no direction from the Supreme Court.

In her letter, the Tamil Nadu Chief Minister has also described the memorandum submitted by the all party delegation from Karnataka to Modi on June 10 opposing the constitution of CMB as "full of misleading statements and distorted facts."