Facebook has come under fire again for blocking a photograph of a 16th-century statue of Neptune, the sea god, because it is "sexually explicit."

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The famed statue stands in Piazza del Nettuno in the Italian city of Bologna and was chosen by local writer and art historian Elisa Barbari to illustrate her Facebook page called "Stories, curiosities and views of Bologna."

But the social media company told Barbari that the picture violated its privacy policies.

In a statement, Facebook said: “The use of the image was not approved because it violates Facebook’s guidelines on advertising. It shows an image with content that is explicitly sexual and which excessively shows the body or unnecessarily concentrates on body parts."

“The use of images or video of nude bodies or plunging necklines is not allowed, even if the use is for artistic or educational reasons.”

The writer, incredulous, responded: “I wanted to promote my page but it seems that for Facebook the picture of our Giant [Neptune] is a sexually explicit image that shows off too much flesh or unnecessarily concentrates on parts of the body. Really, Neptune? This is crazy!"

She later posted on her Facebook page: "Yes to Neptune, no to censorship."

"How can a work of art, our Neptune, be an object of censorship?" she wrote.

“Back in the 1950s, during celebrations for school children graduating, they would cover up Neptune. Maybe Facebook would prefer the statue to be dressed like this."

Created in the 1560s by a Flemish sculptor called Jean de Boulogne — locally nicknamed Giambologna — the statue is one of the most recognisable symbols of the Italian city.

Facebook later told Mashable that the blocking was a mistake: “Our team processes millions of advertising images each week, and in some instances we incorrectly prohibit ads. This image does not violate our ad policies. We apologise for the error and have let the advertiser know we are approving their ad," a Facebook spokesperson said.

In September, Facebook removed an iconic photo from the Vietnam War because it violated its terms of service. When users revolted, the social network reinstated the image and updated its policies.

Facebook also apologised for removing in October a Swedish video on breast cancer awareness and deeming the images offensive.