Apple has been automatically sorting photographs on iPhones for about a year now but one particular category has drawn the ire of twitter users this week.

In 2016, Apple updated its Photos app to recognise more than 4,000 objects, scenes and facial expressions using image recognition.

But one Twitter user this week discovered her iPhone had a "hidden folder" labelled "brassiere".

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The folder contained various shots of what the phone thinks includes bras — but as it happens, those photos were grouped among some that were a little more revealing.

@ellieeewbu discovered the feature when she started typing in the search bar, located in the top right hand corner of the Photos app.

Model Chrissy Teigen also noticed the bizarre feature on her own phone.

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"It's true. If you type in 'brassiere' in the search of your iPhotos, it has a category for every boob or cleavage pic you've ever taken. Why." she asked on Twitter.

"Typing food will get you food but penis won't get you penis and boobs won't get you boobs. Just brassiere. *strokes beard*."

She was also quick to point out that similar items of clothing such as "socks" or "underwear" were not on the list of objects recognised by the phone.

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Although the revelation of a hidden folder has many concerned, Apple's photo sorting happens only on your phone and is not synced to other devices.

And fortunately, the sorting only happens when you enter the search term "brassiere" in the search bar, no physical folder actually exists.

The Photos app, "uses advanced face recognition technology to group photos … [it] even recognises scenes or objects in your photos, so you can search for things like dogs, beaches, mountains or flowers", Apple says on its website.

It was rolled out in June 2016 as an update to the photos app but the official list of terms is not documented by Apple.

The technology also recognises and distinguishes seven different facial expressions: Greedy, disgust, neutral, scream, smiling, surprise and suspicious.

Google Photos has been using a similar feature since 2015, which is probably more of a concern seeing as pictures are both stored and categorised on Google's own servers.

Apple has been contacted for comment.