His announcement came a month after two senior leaders holding cabinet berths - deputy chief minister Roytre C Laloo and Roshan Warjri - expressed similar intentions.
"She wished me well and said I should continue to keep the party united and share my experience and advisories with the workers and leaders," Lapang said in a statement.
He had informed Chief Minister Mukul Sangma about his decision prior to his meeting with Sonia Gandhi yesterday.
Lapang had won the Nongpoh seat in the 1972 assembly elections as an Independent candidate. He later joined the Congress and was elected chief minister in 1992, 2003, 2007 and 2009.
However, he never completed his tenure as the chief minister.
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