The charity warns that Primark is ignoring rising basic living costs for employees making garments in the Bangladeshi capital Dhaka, who are now worse off than two years ago.
It claims that Primark is beating the recession, with profits up 17% to £233m during the 12 months ending in September, while its employees are on the minimum wage, £13.97 (1663 taka) a month, and all of them earning far less than a living wage.
War on Want will stage a protest outside Primark's flagship store in London's Oxford Street this morning with its researcher Khorshed Alam, who has flown to the UK from Bangladesh.
Campaigners from the charity and Alam will then go into the annual meeting of Primark's parent company, Associated British Foods, to speak out against its sweatshops.
The charity's report also reveals similar pay and conditions for Dhaka employees making clothes for Asda, Britain's second-largest clothing retailer by volume, and Tesco, the UK's biggest supermarket fashion chain.
War on Want's protest comes less than six months after the broadcast of Panorama's 'Primark: On the Rack' undercover report on BBC One, exposing the use of child labour to make some of the retailer's clothes.
When presented with the results, Primark sacked three of its suppliers ahead of the broadcast, which attracted 4.2m viewers.
War on Want is demanding that the British government introduce regulation to ensure overseas suppliers pay a living wage and allow exploited staff to seek justice in UK courts.
Employees calculate a worker needs £44.82 (5333 taka) a month to give their family nutritious food, clean water, shelter, clothes, education, health care and transport. Yet average workers' pay, £19.16 a month, is less than half a living wage.
Ruth Tanner, campaigns and policy director at War on Want, said: "Primark, Asda and Tesco promise a living wage for their garment makers. But workers are actually worse off than when we exposed their exploitation two years ago. The UK government must bring in effective regulation to stop British companies profiting from abuse."