Inspectors in the port of Felixstowe are examining 91 containers of rubbish that was allegedly illegally exported to Brazil by two Swindon-based firms.
In July, three men were arrested in the Wiltshire town after Brazil sent a ship carrying the waste back to the UK.
Environment Agency inspectors have found dirty nappies, food and underwear in the containers they have opened so far. The operation is continuing.
It is illegal to export waste from the UK which cannot be recycled.
Andrew Higham, head of the Environment Agency's National Environment Crime Unit, said the containers had to be fumigated for a week before they could be opened.
He said the "substantial'' operation of sorting through the contents had been under way for several weeks and added the agency took a "very serious view'' of the illegal shipment of waste to other countries.
The containers were marked as containing recyclable plastic when they arrived in Brazil, but Mr Higham said all the plastic found so far in the crates was too contaminated to be recycled.