Heroin with a street value of about £5m was found woven into handmade rugs from Pakistan which had arrived at Manchester Airport.
Two men, aged 35 and 51, were arrested after 110lb (50kg) of the Class A drug was found in the consignment that arrived from Peshawar via Abu Dhabi.
A Border Force sniffer dog alerted officers to the drugs.
The pair were held on suspicion of drug importation at a business premises in London and have been released on bail.
Border Force officers and the National Crime Agency's (NCA) Border Policing Command made the seizure and the arrests in April, but the details have only just been disclosed.
A forensic examination of 46 carpets in the shipment is being carried out, and officers said the drugs haul could increase.
Pete Avery, from the NCA's Border Policing Command, said: "It was a sophisticated concealment and demonstrates the lengths organised crime groups go to in an effort to avoid detection."
Linda Paul, assistant director for Border Force, added: "At this stage it is impossible to put an exact value on the drugs because forensic tests have yet to be carried out."