Singh met Congress General Secretary Mallikarjun Kharge on Tuesday and tendered his resignation from the Congress.
He is likely to join the ruling Bharatiya Janata Party soon, though he is reported to be in touch with even the Shiv Sena as well, aides indicated.
Fuelling the speculation to this was the fact that Chief Minister Devendra Fadnavis and Shiv Sena President Uddhav Thackeray had visited Singh's Bandra residence to celebrate Ganeshotsav recently.
Earlier this afternoon, Bollywood actress Urmila Matondkar sent shock-waves in the Congress by quitting after accusing its Mumbai leadership of "betrayal". However, she has not revealed her future course of political action so far though speculation is rife that she may consider joining another party.
Singh - once billed as the Congress' benign north-Indian face in Mumbai - has also expressed his differences with the Congress' official stance opposing the recent striking down of the Constitution's Article 370 in Jammu and Kashmir.
Among other posts, Singh was the Mumbai Congress President for a term and served as Minister of State for Home and held other portfolios in the former Congress-Nationalist Congress Party coalition government.
In 2011, Singh and some of his close family members had come under the scanner of the Enforcement Directorate (ED) and Anti Corruption Bureau (ACB) in certain corruption-related cases after which he had to quit his ministerial post in mid-2012.
Meanwhile, several prominent faces from the Congress and NCP are likely to join the ruling BJP-Shiv Sena this week as the countdown for the announcement of Maharashtra assembly elections starts.
( With inputs from IANS )