India's former External Affairs Minister and former Karnataka Chief Minister S.M. Krishna on Sunday quit the Congress, saying he was being sidelined by its leadership.

"I have resigned from the Congress to protect my self-respect and dignity after serving it for 46 years as the high command has sidelined me because of my age," an emotional 84-year-old Krishna told reporters here.

Clarifying that he was not retiring from politics, Krishna said he had resigned from the party's primary membership through a letter to its President Sonia Gandhi on Saturday.

"I am herewith tendering my resignation (from) the Congress Party," said the letter.

Asserting that age was a state of mind, Krishna regretted that the high command had removed him as the External Affairs Minister in 2012 without any valid reason.

"I did not consult anyone on leaving the party. And except my wife, I did not tell anyone that I was resigning from the party though it is a painful moment in my life," he said.

He said efforts were made by the party's leadership in Delhi to change his mind.

"I am indebted to the leadership which became aware of my existence after I sent the resignation letter. No change in the decision to quit," Krishna said.

Krishna was the External Affairs Minister in the second UPA government from 2009 to 2012 after his stint as the Maharashtra Governor from 2005 to 2008 and Karnataka's Chief Minister from 1999 to 2004.

Krishna was also a Rajya Sabha member.