Some official merchandise for the 2008 Olympics in China has been made using child labour, forced overtime and low wages to boost profits, a report says.
Merchandise is expected to be worth hundreds of millions of dollars
Playfair - an alliance of world trade unions - has condemned "severe workers' rights violations" in four Chinese factories ahead of the Beijing games.
The group said it found abuses at the factories - licensed to make official Olympic caps, bags and stationery.
Companies cited in the report have denied the allegations.
International Olympic Committee said it supported ethical practices.
But in Britain, where the IOC is due to meet on Tuesday, trade unionists said tougher action was needed to make sure that the 2012 Games in London were not tarnished by similar accusations.
Investigators' report - entitled "No medal for the Olympics on labour rights" - cites "gross violations of basic labour standards... including adult wages at half the legal minimum, employment of workers as young as 12 years old".
It also highlights alleged labour rights violations, such as forced overtime, workers being instructed to lie about wages and conditions to outside inspectors and poor health and safety conditions.
BBC visits factories
Guy Ryder, general secretary of the International Trade Union Confederation, said: "It brings shame on the whole Olympics movement that such severe violations of international labour standards are taking place in Olympics-licensed factories."
Chinese labour standards have long been criticised in the West
The IOC said it did not have direct control over merchandise companies - but that host cities were expected to follow guidelines on fair labour standards.
An IOC statement said: "The IOC is committed to being a socially responsible leader of the Olympic Movement that takes care of the Olympic brand in the best way possible.
"It matters to us is that sourcing is done ethically."
The BBC visited two of the factories named in the report, and managers denied the claims.
Lekit Stationery, is a Taiwanese company which has been operating in the city of Dongguan, in Guangdong Province, for the last 20 years. It is making paper cups, notebooks and stickers adorned with Olympic motifs.
The report accused the company of employing children and forcing them to work up to 13 hours a day.
"It's not true," company manager Michael Lee told the BBC. "We work for some of the best brand names in the world and they check our company every month."
He said the factory's 420 workers earned a basic monthly salary of about 700 yuan ($91; £46) and overtime was paid at time and a half.
'Good living environment'
Mainland Headwear Holdings, on the outskirts of the city of Shenzhen, also denied any wrongdoing.
Production director Samuel Wai told the BBC: "We follow all the government regulations here so I'm not sure where these complaints have come from.
"Employees here enjoy a very good living environment and working conditions."
The BBC spoke to some of the employees who live in the apartment buildings outside the factory, who agreed with Mr Wai's assessment.
Another company named in the report was Eagle Leather Products in Guangdong, which makes Olympics-branded bags.
Most workers were obliged to work 30 days per month, the report said, with forced overtime.
The Associated Press news agency reached the factory, speaking to a woman who identified herself only as Ms Chang.
"I cannot agree with the report," she said.
"Our plant is making bags for the Olympics. Our working hours are 8am to 6pm - no extended hours, no child labour."
Sun Weide, a spokesman for the Beijing Olympic Games Organising Committee said he had not read the report, but added: "When we sign an agreement with a firm they have to make a commitment that they will completely honour China's Labour laws and regulations."
China does not allow child labour, he added.