The flood of Chinese imports purchased by Walmart shoppers has cost the U.S. economy about 400,000 jobs since 2001, according to a liberal think tank.

The Economic Policy Institute, which has long been a critic of U.S. trade policies with China, says Walmart (WMT) has been a major factor in the growing trade gap with China. It estimates that Walmart sold at least $49 billion worth of Chinese-made goods between 2001 and 2013, and said that its estimate of 400,000 jobs lost is conservative.

"Walmart has aided China's abuse of labor rights and its violations of internationally recognized norms of fair trade by providing a vast and ever-expanding conduit for the distribution of artificially cheap and subsidized Chinese exports to the United States," the report said.

Walmart doesn't disclose the volume of Chinese imports that it sells. EPI says said it estimated Walmart's Chinese imports using a variety of statistics, including ocean containers going from China to Walmart, and other trade data.

The group estimates that the U.S. has lost a total of 3.2 million U.S. jobs between 2001 and 2013 due to the rise in Chinese imports.

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Walmart, the nation's largest retailer and private sector employer, criticized the study's methodology and conclusions.

"Unfortunately, this is an old report with flawed economic analysis that assumes that imports equal job losses, and does not take into consideration that countless jobs are added through the global supply chain, distribution and logistics, among other areas of the business," said Walmart's statement.

Walmart has long been criticized for selling so many imported goods, and as a result the retailer pledged in 2013 to buy an additional $50 billion of U.S.-made goods to sell in its stores.

But EPI counters that for every U.S. job that program has created, 100 jobs have been lost to Chinese imports.

Walmart defended the success of its buy American program, and points to an estimate from Boston Consulting Group that it will produce 250,000 direct jobs and 750,000 indirect jobs for the U.S.

"We are very proud of our U.S. manufacturing initiative and the results speak for themselves," said Walmart's statement.