Counterfeit currency notes coming into India from Bangladesh through West Bengal's porous borders are the closest to genuine ones and very difficult to detect, according findings of the Reserve Bank of India (RBI).
"A random quality check of counterfeit notes seized across the country is at the heart of this finding," said a source involved in combating the menace.
Experts are worried because producers of the fake notes routed through Bengal have managed to replicate all 12 "overt" security features of genuine notes with a scary degree of accuracy.
"The paper and ink used are of a very high quality too, making it tough even for experts to detect the fakes," an RBI official who did not want to be named said on Wednesday.
The saving grace is, counterfeiters have not managed to incorporate the "covert" security features of genuine notes in their product.
"Only specialists from the apex bank can spot the covert features," the RBI official said.
According to the apex bank's records, 5,21,155 counterfeit notes with a face value of Rs. 2.16 crore were seized in 2011-12.
On the basis of data from different states and central police agencies, the RBI has found over 80% of these notes came through Bengal's Malda, Murshidabad and Nadia districts bordering Bangladesh.
"The 2012-13 trend shows that the figure will be higher," warned the RBI official.