Leaked documents show that Apple is planning to include a 'scent recognition' feature in the iPhone 5 as an additional security measure.

When iPhone 5 owners first set up their handsets, they will be asked to place the iPhone in their underarm, in order to register their scent. Owners can also register the scents of up to five family members or friends who might occasionally use their iPhone.

Once this is done, the iPhone will only work if it can pick up the scent of one of its registered users. Perfume, cologne and deodorant will not affect the iPhone 5's ability to pick up a scent as it is only capable of recognising organic pheromones.

A tiny 'arometer' will sit just by the microphone of the handset, alongside a 1ml tank of indelible ink that will immediately spray any unauthorised user of the iPhone. The ink cannot be washed away or even removed by solvents, making the offender easily recognisable to the police.

The feature was revealed in documents shown to Macworld by an source who wished to remain anonymous but has provided reliable information in the past. A related feature, which may not be available in the first version of iOS 5 but should be added at some stage is 'mood recognition', where the handset will not only be able to recognise individual scents, but the mood of its owner, which is determined by the pungency and intensity of the aroma.

This will be especially useful for those using a new sat-nav app planned for iOS 5 or to play music in their cars via an FM transmitter. If the driver becomes stressed while at the wheel, the sat-nav app will change its tone to offer calming platitudes as well as directions. When playing music, the iPhone will immediately play whale noises and other sounds from nature to calm the driver in high-stress situations.

There are some other clever features built in to the as-yet-unnamed sat-nav app, including aggressive music triggered when the driver is judged to be travelling too slowly to make their destination on time. Slayer's Reign In Blood was the track used in the demonstration seen by Macworld, and it certainly seemed to be effective.

As well as mood, the iPhone 5 will also be able to pick up the smell of drink on your breath, preventing you from making calls - except to Alcoholics Anonymous and The Samaritans - should it consider you to be too drunk. For iPhone 5 owners under the age of 16, the only number it will call is that of the owner's parents.

"In territories where it is illegal to drink alcohol, it will instantly report you to local law enforcement agents," the source told Macworld.

Of course, some of these features are already available on Android handsets, thanks to the Smells Fishy app designed by husband and wife development team Uri and Ivana Laugh. 

UPDATE 1200BST: Sorry - we couldn't resist. Happy April Fools' Day.