"I sincerely apologise to the hardship caused to you by my action on February 13 in the Lok Sabha. Whatever may be the circumstance and reason, I will not justify my action," Rajagopal said in a letter.
The letter was sent on February 18, the very day he sent his resignation as MP to Speaker Meira Kumar. She accepted the resignation the next day.
The expelled Congress leader from Seemandhra, however, said that he was "glad" that many of his colleague MPs understood the "cause for unity of Andhra Pradesh".
Unprecedented pandemonium had broken out in the House last week when Rajagopal, who was elected from Vijayawada, brought in a canister and sprayed pepper.
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All political parties had demanded action against him and the government had said it would not oppose any decision taken by the Speaker.
His resignation came after the Speaker asked the Committee of Privileges of Lok Sabha, which has punitive powers, to go into the incident which she termed as a "blot" on democracy.