Dara Singh Chauhan - one of nearly a dozen ex-BJP MLAs, including three ministers, to quit the party this week ahead of next month's election - joined Akhilesh Yadav's Samajwadi Party on Sunday.
"In 2017, the BJP government gave a slogan of 'sabka saath, sabka vikas'. The party took support from everyone but only few people got the benefits of development," he said.
"We will change politics of UP and make Akhilesh Yadav Chief Minister again. People belonging to OBC and Dalit communities will come together... a change is inevitable," he added.
Apna Dal MLA RK Verma, whose party is allied with the BJP, also joined the Samajwadi Party today.
"I welcome Dara Singh Chauhan, RK Verma. This (the 2022 election) is a fight with the double engine government in Delhi and Lucknow (a reference to the BJP in power at centre and state). They have only done 'politics of breaking'. We will focus on 'politics of development'," Mr Yadav said.
Mr Chauhan is possibly the last major opposition leader to be signed by Mr Yadav, who has snapped up all three of the ministers who quit, after he said yesterday "we will not take any more leaders".
That was after seat-sharing talks with Chandrashekhar Azad broke down.
An influential OBC leader who has also served as a Lok Sabha and Rajya Sabha MP, Mr Chauhan was the Environment and Forests Minister in Chief Minister Adityanath's cabinet till he quit on Wednesday.
His resignation (and those of the other MLAs) was seen as blowing a big hole in the BJP's re-election bid, given the party relied on non-Yadav OBC votes to defeat Mr Yadav in 2017.
He, like almost all of those who quit, blamed the BJP for ignoring the needs of UP's backward classes; "I am hurt by this government's oppressive attitude towards the backward sections..." he said.
Ex-minister and key OBC leader Swami Prasad Maurya began the flood of defections Tuesday, telling NDTV the BJP's UP government was "deaf to the problems of the backward classes".
Swami Prasad Maurya was inducted yesterday, with another ex-minister in Dharam Singh Saini (who quit the BJP 24 hours after saying he would not) and six MLAs, including another from the Apna Dal.
For Mr Yadav, a former UP Chief Minister seen as the BJP's main rival for this election, the acquisition of these OBC leaders is a big boost as he looks to replicate its 2017 formula.
Last year, another influential OBC face and a former BJP ally, Omprakash Rajbhar and his Suheldev Bharatiya Samaj Party, crossed over to the Samajwadi Party.
In November Mr Yadav told NDTV a "pincer" movement of angry farmers in the west and a "rainbow alliance" of regional parties in the east would "wipe out the BJP" in the election.
UP will vote for a new government in a seven-phase poll that begins February 10.
Yesterday the BJP said Chief Minister Adityanath would contest from Gorakhpur. Mr Yadav has not confirmed if he will stand for election; in November he told NDTV he would do so "if the party wants".
With input from ANI, PTI