A Border Force sniffer dog alerted police at Manchester Airport to Pakistani rugs that had heroin worth over £5 million sown into them.
The massive haul of the class A drug, which weighed 50kgs, was found in a consignment that arrived from Peshawar via a flight from Abu Dhabi.
Forensics are combing over the carpets and it is possible the amount of heroin contained with them could increase and with it the street value of the drug.
Two men, aged 35 and 51, were arrested after the discovery and held on suspicion of drug importation. They have since been released on bail.
Border Force officers joined the National Crime Agency to make the seizures and arrests, which happened in April but details have only just emerged.
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, said: "This is an extremely impressive seizure and I would like to praise the vigilance and hard work of my officers who have ensured that a substantial amount of harmful drugs have been kept off our streets."