Here are the latest developments in this big story:
The resignation came after Chief Minister Akhilesh Yadav refused to return key portfolios to Shivpal Yadav that he had been divested of earlier this week, sources said.
On Thursday, Samajwadi Party chief Mulayam Singh came down from Delhi in a bid to find a solution to the crisis. Though he is living in the same house with the Chief Minister, it is not known if the two have met.
Mulayam Singh has already met Shivpal, his favourite brother, twice, to hear his part of the story. On Wednesday, Shivpal had rushed off to Delhi for a four-hour meeting. The two met again this afternoon after Mulayam Singh reached Lucknow.
The rift within had been growing for months but the crisis came on Tuesday, hours after Mulayam Singh removed Akhilesh as the state party chief and appointed Shivpal. The move was widely seen as a punishment for the Chief Minister's run-in with Shivpal.
Late on Tuesday evening, the Chief Minister had divested Shivpal Yadav - a key minister in the state government - of all his crucial portfolios, leaving him with only the social welfare ministry.
The leaders claim there is no dispute within the family and call the feud a "conspiracy" by outsiders.
Without naming anyone, Akhilesh Yadav has said, "If people from outside the family keep interfering, how will things work?" Many interpret the statement as a reference to Amar Singh, who returned to the Samajwadi Party recently after a six-year exile.
Many party leaders believe that Amar Singh is in league with Shivpal Yadav in a plan to turn Mulayam Singh against his son. It is also believed that he had engineered the shifting of Akhilesh Yadav from the post of the state party chief.
The feud, five months before the elections has drawn opposition jibes. The Chief Minister, Congress vice-president Rahul Gandhi said, was trying to change the image of his government by "replacing tyres of his damaged cycle" but it was too late.
Calling the whole episode a "drama in the absence of any tangible work done by the SP government," BSP chief Mayawati advised Mulayam Singh to retire.
Amid Reports Of Links With IPL Side, VVS Laxman Likely To Extend Stay At NCA Video: Patients Bang OPD Doors, Doctors Raise 'Justice' Slogans In UP Virat Kohli, MS Dhoni, Rohit Sharma Unite For Noble Cause, Donate For Charity Auction Parents Gave Names Of Interns, Doctors To CBI In Kolkata Horror: Sources Who Is Jasveen Sangha, "Ketamine Queen" Charged With Matthew Perry's Death How Extreme Weather Is Leading To Rise In Child Marriages In Pakistan 80 Killed In Paramilitary Attack In Sudan OpenAI Blocks Iranian Group's ChatGPT Accounts For Targeting US Election Blinken Heading To Israel To Press Gaza Truce Track Latest News Live on NDTV.com and get news updates from India and around the world.