Essex Police said there were initial concerns more people could be inside a container that arrived at Purfleet but that this turned out not to be the case.
Belgian police said they believed the lorry which delivered the container in Zeebrugge had been identified through CCTV footage.
Chief Inspector Peter De Waele said it was likely the people were already inside the container when it was dropped at Zeebrugge as it appeared "impossible" the group could have entered it during the hour it was at the port.Natural stone
It is not known where the container, one of 64 aboard the P&O commercial vessel Norstream, originated, nor where the people inside it were heading.
'Exploited by gangs'
Former head of the UK Border Force Tony Smith said those inside the container were victims of international organised criminals.
He told the BBC: "They're being exploited because the prize is a passage to the West - that's what they want, they want to migrate to the UK or to Europe but they're being exploited by criminal gangs who are probably taking their entire life savings away on the promise of a passage to the West.baby product
"We really need to get a message out to migrants that if they want to come to this country there are legal routes that they need to explore and they need to apply for visas and permits."
Anthony Steen, chairman of the Human Trafficking Foundation, said: "It shows how desperate people are to improve their economic situation - how desperate they are to leave their own homes, and own countries, and hope to arrive in somewhere that's more accommodating, more kind, and offering them a better quality of life. Usually, they're sadly wrong."
Police have set up a "casualty bureau" hotline for anyone concerned about relatives. The numbers are 0800 056 0944 or 0207 158 0010 if dialling from outside the UKusb dac