The container, which was spotted at Axmouth, near Seaton in Devon, at 8.11am yesterday, is believed to be carrying 14 tonnes of cigarettes.
It is thought to be one of many lost from a Maersk cargo ship as it crossed the northern stretch of the Bay of Biscay last Friday.
Police and the Maritime and Coastguard Agency have issued a stern warning to treasure hunters planning to benefit from the haul. A spokesman for Devon and Cornwall Police said: "The contents belong initially to the original owner of the container, and then to the official Receiver of Wrecks, so anyone taking anything that washes ashore, even one packet of cigarettes, is effectively committing theft and will be prosecuted.
"It is not a free-for-all for the public. There are dangers associated with containers and products which have washed ashore. The message is, police are seizing them and would ask the public to steer clear."