A routine check on an unnamed Maersk containership in Mexico uncovered 23 tonnes of the deadly painkiller fentanyl.
Mexican media said the drug was contained in 931 bags disguised as a cargo of calcium chloride, which is used in road salt.
The ship had arrived from China last month.
The painkiller is up to 50 times stronger than heroin and is responsible for a growing number of deaths worldwide.
US Customs has said the drugs were probably headed to the US.
Maersk told Ekstra Bladet said: "Maersk does not accept illegal cargoes and has incorporated firm guidelines and procedures, for example. for inspecting and handling loads that come in already from the actual ordering of a container.
"We also work proactively with the authorities to limit the risk of getting illegal cargo on board and of course fully cooperate with relevant authorities when investigations are ongoing."
