Image copyright Forever 21

US fashion retailer Forever 21 is in hot water after sending out free diet bars with online orders of clothing.

Customers flocked to social media complaining the retailer was "fat-shaming" women after they received Atkin bars with plus-sized orders.

"What are you trying to tell me Forever 21 - that I'm fat, lose weight?" asked one angry customer on Twitter.

The retailer has responded, apologising for the "oversight" and insisting that all customers were sent the snack bars.

"From time to time, Forever 21 surprises our customers with free test products from third parties in their e-commerce orders," Forever 21 said in a statement.

"The freebie items in question were included in all online orders, across all sizes and categories, for a limited time and have since been removed.

"This was an oversight on our part and we sincerely apologise for any offence this may have caused to our customers, as this was not our intention in any way."

Samantha Puc, the managing editor of comic news site The Beat, warned Forever 21 that it was sending "a wildly dangerous message" to customers who might have eating disorders.

However, other users, who had received the diet bars with orders of clothing in smaller sizes, chided those who were upset on Twitter, saying they should "pick their battles better".

Forever 21 is no stranger to controversy. Several fashion designers, including Anna Sui, Gucci, Diane Von Furstenberg and Anthropologie, have previously sued the retailer over copyright and trademark infringement.

The retailer has also been criticised online for allegedly "stealing" art from Instagram and copying the work of independent designers.