She won the seat in West Bengal's Jalpaiguri district in 2016 on a TMC ticket.
Former TMC MLA of Dhupguri Mitali Roy on Sunday morning joined the BJP, two days before the by-election in the seat.
She won the seat in West Bengal's Jalpaiguri district in 2016 on a TMC ticket, but lost to BJP's Bishnu Pada Roy in the 2021 assembly elections.
It was the BJP MLA's death on July 25 that necessitated the by-election, the polling for which will be held on Tuesday. The votes will be counted on September 8.
Ms Roy joined the BJP in the presence of its state president Sukanta Majumdar, Jalpaiguri MP Jayanta Roy, Dabgram-Phulbari MLA Shikha Chatterjee and the party's district president Bapi Goswami.
"I was not being able to work in the TMC. I was under tremendous mental pressure. I did not want to campaign for the Dhupguri bypoll but I was being forced for it. I joined the BJP because it is in power at the Centre, which will help me to bring development to the area and fulfill the people's needs," she said.
Mr Majumdar said Ms Roy is a veteran leader of the area, and she knows about the needs of the people.
"Her joining will strengthen the BJP," he said.
On Saturday, BJP's former district president Dwipen Pramanik joined the TMC.
The TMC has fielded college professor Nirmal Chandra Roy in the seat for the bypoll, while the BJP has nominated Tapasi Roy, the widow of a CRPF jawan killed in a terrorist attack in Jammu and Kashmir in 2021.
The CPI(M) has fielded folk singer Ishwar Chandra Roy and the Congress will extend support to him.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)