"I saw the letter this evening. It was sent by post from Kharagpur in West Midnapore district on May 13 by N Rama Rao of Indian People's Liberation Army," Ghatak told PTI.
Chief Minister Mamata Banerjee and the police have been informed about the matter, he said.
"The letter asked me to quit the party and resign from the Cabinet because of my closeness to Mamata Banerjee who they allege had a key role in the killing of Maoist leader Kishenji," Ghatak said.