"Ninety-nine percent of the raw materials that are used through the internet to make up in your kitchen or your backyard laboratory are emanating from China," Howman said in an interview with Reuters television during a WADA media symposium at a London hotel.
"They are going to gangs who then put them together in little plastic capsules and are sold on the street or in the gyms."
Two years ago Howman caused a stir at a doping conference convented by the World Sports Law Report, also in London, when he said the criminal underworld controlled a large proportion of world sport.
On Tuesday, he said he thought at least 25 per cent of international sport was now controlled "to one degree or another" by the underworld.
"I am making this statement on information I have received, this is not a guess, it's probably even higher," Howman said.
"And as we go on I think you will see that increase. That's a concern, because that hits at the heart and the integrity of sport that we have grown up with.
"The black markets supply a lot of pharmaceutical products before they are out on the white market. That's run by the criminal underworld, so a lot of the pharmaceutical stuff comes out in that way.
"There's all the stuff we talk about in terms of the kitchen laboratories or the backyard laboratories which are assembled through the criminal market."
Howman said the criminal gangs involved in supplying illegal drugs were also involved in match-fixing. Europol announced this month that as many as 380 football games across Europe had been involved in a match-fixing inquiry.
"It's the same bad guys, we know this because we have been involved in investigations and involved with people who are in this area on a daily basis," Howman said.
"It's the same bad guys pushing the drugs, laundering their money through regular betting and legal betting, bribery and corruption."
Earlier the New Zealand lawyer told the symposium that more money was made out of selling drugs to high school children than to elite athletes.
"I can just tell you a personal story," he told Reuters. "In Montreal (where WADA is based) I was told that if I wanted steroids I should just go along to the local gym and the guy in the corner dishing them out. Now the people who are receiving these drugs are not elite athletes.
"They are either high school kids who want to look good for whatever reason, security people who want to pump up or the older people who want to do anti-ageing."
Howman also said there was a huge unregulated dietary supplement industry in the United States where the substances were unlabelled.
"Most of the supplements around the world that you see come from the United States. And we know only too well because we have seen some of the more recent supplements intentionally include performance-enhancing components because they want to get a bit of an edge with the markets that they are servicing," he said.
- Sports & Recreation