Sending toxic waste abroad is illegal under international law
Around 1,500 tons of hazardous waste which arrived in Brazil from the UK labelled as recyclable plastic is on its way back, authorities have said.
The Brazilian Institute of Environment and Renewable Natural Resources said the cargo included used syringes, condoms and dirty nappies.
Three British men have been arrested over alleged illegal shipments.
Officers from the Environment Agency and Wiltshire Police raided three properties in Swindon in July.
Three men, aged 24, 28 and 49, were bailed until the end of October.
The Institute said 81 containers arrived between February and May 2009 at Brazilian ports.
It added that they were labelled recyclable plastic, but had been packed with domestic and hospital waste.
They were sent back towards the UK from the port of Santos on Wednesday.
The Environment Agency said it would investigate their contents before disposing of them safely.
Brazil's government said it would formally complain to the World Trade Organisation over the deliveries.
Under the Basel Convention, shipments of toxic waste from industrialised nations are banned, officials said.
Waste can be exported for recycling, but it is illegal to send it abroad for disposal.
The maximum penalty for doing so is an unlimited fine or up to two years in jail.