Shashank's removal is the only solution for smooth conduct of the November 28 Assembly elections, the chief minister said in the letter.
He said people of Mizoram have lost faith in Shashank, who, according to a section of media, complained to the Election Commission (EC) that principal secretary (home) Lalnunmawia Chuaungo was interfering in the poll process.
Chuaungo, a 1987-batch Gujarat-cadre IAS officer, has been relieved of his responsibilities in the state.
The EC has instructed him to report before the Ministry of Home Affairs in Delhi.
"As people have lost faith in him, the only solution for smooth conduct of Assembly elections 2018 now would be removal of CEO S B Shashank from office forthwith," the chief minister wrote in the letter, a copy of which is available with PTI.
"Additional CEO can take over Shashank's responsibilities if making fresh arrangements were difficult at this juncture," the CM said.
Earlier in the day, Shashank told reporters here that he was fulfilling his responsibilities in accordance with the poll panel's instructions and sought an apology for "hurting sentiments" of the people of the state.
NGO Coordination Committee, an umbrella body of major civil societies and student organisations, sought Shashank's exit from the state, shortly after the EC removed Chuaungo.
The committee also threatened to picket the state election office if Shashank does not leave the state by Monday evening.
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