IT'S the craze that's swept social media but in the past 24 hours worries over using the app have spread even quicker.

If you have spent any time on Facebook, Twitter or Instagram this week, you will no doubt have spotted at least one friend or celebrity has suddenly aged and used the hashtag #FaceAppChallenge.

FaceApp is a state-of-the-art photo editor powered by artificial intelligence.

The free app allows you to apply a variety of filters to your selfies to see how you would look with glasses, a beard or different coloured hair.

But by far the most popular has been the old age filter, transforming faces to look like they are 40 or 50 years older.

While some have taken to Twitter to show others how they would look in 2070, many others have warned about the app's terms and conditions.

The terms and conditions state: "You grant FaceApp a perpetual, irrevocable, nonexclusive, royalty-free, worldwide, fully-paid, transferable sub-licensable license to use, reproduce, modify, adapt, publish, translate, create derivative works from, distribute, publicly perform and display your User Content."

A number of other online privacy experts have raised concerns that the app, developed by a Russian company, could be accessing and storing users' images without their permission.

While others have warned users should be wary of using apps which grant photo access if they have screenshots on their phone of sensitive information including banking details.

Gordon Ramsay is one of the many celebs who have used the app

FaceApp has since confirmed most images were deleted from its servers within 48 hours of being uploaded and it only ever uploaded photos that users selected for editing and not additional images.

Company bosses have also told reporters they don't use photos for facial recognition training and only for editing pictures.

Others have questioned why FaceApp needs to upload photos to a cloud when they just process the images locally on the user's smartphones.

But experts say it makes sense if they want to have a competitive advantage.

What should I do if I've already used the app and I'm worried?

A statement from FaceApp adds that while they accept requests from users to have their data deleted, the company's support team was currently 'overloaded'.

It added that data was not transferred to Russia.

FaceApp advises users to submit such requests through settings, support, 'report a bug' and add 'privacy' in the subject line.

Other experts have argued the terms of using the app are very similar to other social media sites including Twitter.

The UK's Information Commissioner's Office (ICO) has issued the following advice: "We would advise people signing up to any app to check what will happen to their personal information and not to provide any personal details until they are clear about how they will be used."