The seized notes in Lalbazar. (Pradip Sanyal)

Police on Friday seized fake notes of Rs 50 lakh — the biggest ever counterfeit currency haul in the city — from a gang of four who had put up in a central Calcutta guest house.

“Pakistan’s ISI had sent the notes which were to be pumped into select banks and financial institutions as well as projects undertaken under the National Rural Employment Guarantee Scheme in south Bengal,” said Rajeev Kumar, the special additional commissioner (II) of police.

A team of officers from the Special Task Force raided a room in the guest house following a tip-off and found a bag stashed with fake notes, mostly in the denomination of Rs 500 and Rs 1,000.

The four men who were acting as couriers have been arrested. Two of them — Upendra Chaudhury and Kumar Gautam — are Nepali citizens.

“Chaudhury and Gautam are close to Yunus Ansari, the son of a minister in Nepal, who is now behind bars for his links with a fake currency racket,” said an STF officer.

Ansari and Gautam also worked with Munna Khan, an alleged ISI operative in Kathmandu involved in pushing fake notes into India. Munna is absconding.

The other two in the net are Naushad Khan and Zafar Ali Mollah, residents of Motihari in Bihar.

The racket had earlier pumped a substantial number of fake notes into the Terai region. “These notes, smuggled in with help from some Nepali officials, made their way into projects under the rural employment scheme in north Bengal and north Bihar. Now, the racket seems to have turned its focus on south Bengal,” said an officer.

Interrogation of the quartet revealed that the fake currency racket had set up base in four cities abroad — Kuala Lumpur, Bangkok, Kathmandu and Dhaka — in order to flood India, said an officer.

“The ISI prefers to send consignments by air from Kuala Lumpur and Bangkok, while the road route is a safe corridor for delivery from Kathmandu and Dhaka,” he added.

Fake notes of Rs 1.5 crore — headed for India — were seized in Malaysia last month. Experts who examined the notes said the ISI had cracked almost all security codes.