Biffa has been convicted of breaking the law by sending household rubbish to China that was labelled as waste paper.
Instead, the bundles included nappies, sanitary towels and condoms, according to the Environment Agency, which prosecuted the waste giant.
The waste management firm was found guilty in a three-week jury trial at London’s Wood Green Crown Court.
Biffa said it “strongly contested” the decision and was considering making an appeal.
But the Environment Agency said jurors did not accept Biffa’s claim that the contaminated bundles were made up of waste paper.
Biffa argued that its containers were regularly inspected by Chinese customs agents. It also said the firms buying the waste often inspected containers before they were shipped to make sure they contained 98.5% paper, which is the industry standard.
It has been illegal to send unsorted household waste to China since 2006.
Paper can legally be sent to the country, but other heavily contaminated waste cannot.
The Environment Agency said it found “everything from women’s underwear and plastic bottles to metal pipes” in a number of 25-tonne containers that were bound for China.
Glass, plastics, electrical items and metal were also found inside seven of the containers that the agency inspected at the port of Felixstowe in Suffolk.
“Instead of waste paper, investigators discovered diverse discarded debris such as shoes, plastic bags, an umbrella, socks, hand towels, unused condoms, video tape, toiletries and electric cable,” the Environment Agency said.
“The nappies and sanitary towels gave off a pungent ‘vomit-like’ smell when inspected by Environment Agency officers.”
But Biffa said it supplied “vital raw material” to China to be recycled in an “environmentally sound” way.
It said the material met international standards and blamed the Environment Agency for failing to lay out what level of contamination it would consider acceptable.