The 'totally undetectable' app that lets you spy on friends, family or colleagues

It's a secret agent's dream: one single piece of software that lets you into all of a 'target's communications, their movements and their personal notes.

But this isn't some piece of top-secret NSA infrastructure. mSpy is a smartphone app that works on Android, Apple, Blackberry and Nokia phones - offering a staggering array of surveillance options.

The app - which works on a subscription basis starting at £24.99 a month - is described as being able to 'run undetected on your child's or employee's cell phone and provide all of the necessary features for complete monitoring.'

Even in its most basic package, the app offers users a way to spy on phone calls, texts and emails, as well as providing a notification if the phone's owner switches SIM cards in an attempt to avoid detection.

For £44.99 a month, mSpy adds the ability to monitor communications on third-party apps, including Skype, Facebook, WhatsApp and iMessage.

Both versions of the app allow the user to see the phone's internet browsing history, what applications it has installed, and view photos or videos taken. Even if files are deleted from the phone, they can be saved to a laptop or PC.

Perhaps of most use to would-be snoopers is the location-tracking ability. mSpy can be set to sync over 3G, and provide Google maps results showing the phone's location.

It uses GPS or, when that isn't available, 'Cell ID technology' that uses the mobile phone's network to pinpoint its location. It's possible to look back at the phone's movements over any period of time.

mSpy comes with software that gives graphs and charts of the target smartphone's usage, so users can see at a glance if there is any suspicious activity at odd times of the day or night. Users can see which apps have been used and when, and keep track of the phone's battery life.

Some of the app's scariest features are only available to premium customers paying the higher monthly fee. The app will log every keystroke recorded by the phone - including passwords - and lets the owner record phone calls without the target's knowledge.

[The bigger your smartphone, the more data you use]

Using the phone's microphone, it can even be used to record the phone's surroundings surreptitiously. If you want, you can block incoming calls or restrict access to certain websites. As a last-ditch measure, mSpy allows you to remotely lock or wipe the phone in an instant.

mSpy has been used by spouses to catch cheating partners; by parents keeping track of their children, and by bosses who want to keep tabs on their employees. Deep in the app's small print it notes that 'You are required to notify users of the device that they are being monitored' - but that is clearly at odds with many of the featured case studies.

Nick Pickles, director of Big Brother Watch, said of the app that: 'The fact companies are selling extremely intrusive spyware so brazenly begs the question if we are properly protecting people from snooping by their boss, parents or indeed anyone who has enough time to install the app on their phone.

'This is the kind of technology you'd expect the security services to be using, not for sale with the click of a mouse. You don’t teach kids to think about their privacy by secretly listening to their calls or reading their emails, nor do you create a productive working environment by routinely snooping on your employees.

'The risks of abuse are only too clear and this kind of technology needs to be much more tightly regulated,' he added, speaking to MailOnline.

It is necessary to physically install the app on the phone you would be monitoring, and iPhones or iPads would need to be 'jailbroken' - a process that enables the installation of apps from sources other than the App Store. It doesn't run on iOS7, and Android phones also need to be 'rooted' - allowing access normally denied to the phone's user.