Briefing the media, party Vice President Kironmoy Nanda said the announcement had the approval of party chief Mulayam Singh Yadav.
Last week, Mulayam Singh had said that legislators would decide the next Chief Minister after the elections due early next year.
This had triggered speculation that Chief Minister Akhilesh Yadav had lost the goodwill and support of his father and party chief.
Soon after getting the letter, Mulayam Singh met Ram Gopal Yadav in New Delhi for some three hours.
Mulayam Singh, his brother Shivpal Yadav and Akhilesh Yadav have been at daggers drawn over power sharing over the past one month.
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