The special CBI judge hearing arrested senior TMC leader Anubrata Mondal's case has received a letter threatening to implicate him and his family in a drug case if the politician is not granted bail soon, a source said on Tuesday.

Asansol's CBI court judge Rajesh Chakraborty has written a letter to the district judge of Paschim Bardhaman district requesting him to take note of the "threat" and bring it to the notice of the registrar, Judicial Service, Appellate Side of the Calcutta High Court.

With great anxiety, I do hereby forward to you a letter by one Bappa Chatterjee being addressed to the Officer-in-charge of this court threatening that if Anubrata Mondal is not released, my family members would be implicated in NDPS case with commercial quantity, the judge's letter read.

The CBI arrested Mondal, the Birbhum district president of the Trinamool Congress, in a cattle smuggling case on August 11.

Mondal had appealed for bail on August 20 citing poor health, but the CBI counsel opposed his plea describing him as a very powerful and highly influential person . If granted bail, he could influence witnesses and tamper with evidence , the central agency said.

The judge, who then extended Mondal's CBI custody till August 24, got the threat letter also on August 20.

The special CBI court in Asansol is hearing the cases of coal and cattle smuggling. Several people were arrested in these cases.