The party has filed a petition before the Speaker seeking disqualification of Shivpal Singh Yadav from the Assembly.
Shivpal Yadav, who was elected to the state Assembly on the SP ticket from Jaswantnagar in 2017, now heads the Pragatisheel Samajwadi Party-Lohia (PSPL) that was floated by him in October last year.
The Samajwadi Party, however, had avoided any action against him, apparently keeping the doors open for a rapprochement.
Samajwadi patriarch Mulayam Singh Yadav had been constantly trying to bring about a patch-up between his son and SP president Akhilesh Yadav and brother Shivpal.
Sources said that Akhilesh was unwilling to build bridges with his estranged uncle, but postponed any action under pressure from his father.
However, late on Thursday night, the Uttar Pradesh Legislative Assembly secretariat issued a statement saying that the Leader of Opposition and SP leader Ram Govind Chaudhary had moved a petition seeking cancellation of Shivpal's membership on the basis of 'disqualification on the ground of defection rules, 1987'.
The petition states that Shivpal Yadav had quit the Samajwadi Party on his own last year and he first formed the Samajwadi Secular Morcha (SSM) and then registered a political party -- Pragatisheel Samajwadi Party-Lohia (PSP-L).
PSP-L contested the Lok Sabha polls across the state. Shivpal himself contested the Ferozabad seat against the party's sitting MP Akshaya Yadav.