Law minister Chandrima Bhattacharya said Mitra's resignation was accepted by Chief Minister Mamata Banerjee.
Mitra was replaced by Bimal Chatterjee, a senior barrister, on the instruction of the chief minister, she said.
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Asked why Mitra resigned, she said the state government was unaware of it.
Bhattacharya also replied in the negative when asked if there was any complaint that Mitra was not provided with proper evidence and documents in major legal cases he was dealing with.
There was no complaint of non-cooperation with Mitra by any state government department, the minister said, adding that the state government had no problem with him.