Apologising for banning an ad that showed a model wearing a bikini, social media giant Facebook has reversed its decision that it took claiming the image in the ad was a violation of the company's health and safety standards.

The social media site banned the photo of Tess Holliday -- a plus-size model -- saying that the photo was against the guidelines that prohibit displaying "body parts in an undesirable manner", the New York Post reported.

"Our team processes millions of advertising images each week and in some instances we incorrectly prohibit ads," Facebook was quoted as saying.

"This image does not violate our ad policies. We apologise for the error and have let the advertiser know we are approving their ad," the company added.

Recently, Facebook refused to promote a conference to be hosted in June by a feminist Australian group.

The group even tried to pay the company to boost the post to get more people to register but Facebook denied it saying that they would have to remove the image and resubmit the event for approval.

"Ads may not depict a state of health or body weight as being perfect or extremely undesirable," Facebook wrote, adding, "Ads like these are not allowed since they make viewers feel bad about themselves. Instead, we recommend using an image of a relevant activity, such as running or riding a bike."