West Bengal Chief Minister Mamata Banerjee was on Thursday administered the oath as a member of the state assembly by Governor Jagdeep Dhankhar.
Ms Banerjee, who won the Bhabanipur by-poll by a margin of 58,835 votes, a record for the seat, took the oath in Bengali.
She needed to win the election to retain the chief minister's chair after her defeat at the hands of protege-turned-rival BJP's Suvendu Adhikari in the March-April assembly elections.
Two other newly elected TMC legislators Jakir Hossain and Amirul Islam also took oath soon afterwards.
Jakir Hossain was declared victorious by a huge margin of 92,480 votes from Jangipur, Amirul Islam won from Samserganj by 26,379 votes.
Mr Dhankhar had on Tuesday said he will administer the oath to Ms Banerjee and others, in a departure from the practice of the governor authorising the speaker of the assembly to conduct the exercise.
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