A man has died after 35 people - including children - were found in a shipping container at Tilbury Docks.
The survivors - believed to be from the Indian subcontinent and suffering from severe dehydration and hypothermia - are being treated at nearby hospitals.
The discovery was made on Saturday when the ship, which had arrived from Zeebrugge, Belgium, was being unloaded, reports BBC.
Essex Police has launched a homicide investigation and officers are being assisted by their Belgian counterparts.
Police Supt Trevor Roe said staff at the docks were alerted to the container by "screaming and banging" from inside.
He said the other containers on the freighter were being searched to make sure there were no other people inside.
It is not known where the container, one of 64 aboard the P&O vessel, originated.
The East of England Ambulance service, which was called to the docks at 06:37 BST, sent seven ambulances, two rapid response cars, two doctors and a hazardous area response team to the scene.
There is a police presence at Basildon Hospital, part of which has been taped off. Eighteen people from the container are being treated there.
On its website, the hospital said that its accident and emergency department was "responding to a major incident".
Seven patients were taken to Southend Hospital while nine were taken to the Royal London Hospital, Whitechapel.
'NOISE FROM CONTAINER'
The ship involved was a P&O freighter called the Norstream. The container was loaded on to it at about 21:30 BST on Friday at Zeebrugge.
A P&O spokeswoman said the Norstream, which was scheduled to leave Zeebrugge at 22:00 BST on Friday, was carrying 64 containers, 72 trailers and five lorries and their drivers.
She said the discovery was made when "noise from the container made it apparent there were people inside."
Belgian police are assisting Essex Police with their investigation and examining CCTV footage as part of their enquiries.
Public Health England said it was not currently involved and had not been notified of there being any Ebola risk.
A spokeswoman for the port declined to comment on the incident as it "was a matter for the police and Border Force".
The ship was sailing on a new service linking Tilbury and Zeebrugge which has only been operational since earlier this month, according to parent company Forth Ports.