Apple's default Photos app sorts users' photos into different categories – from "dogs" to "weddings" to "brassieres" - Representational image Reuters/Bobby Yip

A Twitter user has found that simply searching for the word "brassiere" in Apple's default Photos app on iPhones yields photos featuring bras that that are automatically categorised into a group. Twitter user @ellieeewbu called attention to the feature this week, saying, "ATTENTION ALL GIRLS ALL GIRLS!!! Go to your photos and type in the 'Brassiere'. Why is apple saving these and made it into a folder!!?!!?"

The discovery immediately drew surprise and criticism on social media over privacy concerns, with many voicing apprehensions that Apple may be saving these images. However, Apple's default photo app analyses content and creates categories, not folders, to allow users to sort through and view their images using Photos searches.

Apple Photos has been using image-recognition technology and metadata analysis since June 2016 to categorise users' photos automatically and tag them to allow for quicker and easier photo searches.

According to developer Kenny Yin, Apple's Photos app supported detecting 4,432 different objects and scenes which can be searched for in all languages as of June last year.

"The Photos app makes it easy to find photos of an exact person, place, or thing. Advanced face recognition and computer vision technology lets you search your photos by who and what's in them," Apple's support page reads.

"Photos recognises scenes or specific objects in your photos, so you can search for things like dogs, mountains, or flowers. When you search your photos, all of the face recognition and scene and object detection are done completely on your device."

Many Twitter users testing out the feature reported that the image recognition technology seemed to generate a wide range of photos that it interpreted as containing the object, theme or event in question. While searching for "brassiere", some of the photos that popped up included dresses with skinny straps, tank tops, swim suits, sports bras and bras.

A number of users posted screenshots of their photo searches that returned images which qualified under the "brassiere" category. In some cases, it churned out nude images or pictures of them having sex.

Others tested out Apple's algorithms using other keywords such as dog, cat, wedding and beach. However, some people noted that it did not seem to include any other types of undergarments, particularly male equivalents, such as "underwear", "boxers" or "briefs".

Apple has yet to publicly respond or clarify the issue.