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Following reports that the Apple Watch doesn't work properly when worn on tattooed wrists, the tech giant has confirmed the issue.

On the official support page for the Watch's heart rate sensor, Apple wrote: 'Permanent or temporary changes to your skin, such as some tattoos, can impact heart rate sensor performance.'

And the retailer is advising people to use external heart rate monitors as a workaround for the problem.

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Some owners of the Apple Watch have found that it malfunctions if worn on tattooed wrists (pictured).  Apple confirmed that 'permanent or temporary changes to your skin, such as some tattoos, can impact heart rate sensor performance' on an official support page

The issue first came to light on Reddit and Twitter where a number of users reported problems, such as having to repeatedly enter a password to unlock the phone.

This is required every time the device detects it has left a wearer's wrist.

It appears the pigmented ink of some tattoos affects the watch's sensor's ability to read an individual's heart rate, which in turn leads the watch to think it has stopped making contact with the skin.

There is also speculation that the fault could stop some people using Apple Pay.

Under the heading, 'What else affects your reading?' on the support page, Apple said: 'The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.

'If you're not able to get a consistent reading because of any of these factors, you can connect your Apple Watch wirelessly to external heart rate monitors such as Bluetooth chest straps.'

It stressed that heart rate is just 'one of many factors that Apple Watch uses to measure your activity and exercise' and depending on the workout the Watch selects the most appropriate inputs for that activity. 

For example, when running indoors, it uses the accelerometer, and when cycling outdoors, it uses the GPS in your iPhone. 

Tests conducted by iMore suggest that tattoos in dark and solid colours interfere with the device the most. 

This is because the heart sensor reads a pulse by measuring light absorption though the skin - a process called photoplethysmography.

'This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light, Apple writes on its support page. 

'Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. 

'When your heart beats, the blood flow in your wrist - and the green light absorption - is greater. Between beats, it's less. 

'By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute - your heart rate. 

So the problem seems to occur when a wearer has something on their skin that reduces the lights reflectiveness, such as ink under the skin's surface.

It appears the pigmented ink of some tattoos affects the watch's sensor's (illustrated) ability to read a heart rate, which in turn leads the watch to think it's stopped making contact with skin. Apple added, 'if you're not able to get a consistent reading, connect your Apple Watch to external heart rate monitors'

This doesn't include natural skin pigmentation.

iMore tested the watch's sensors on tattooed and non-tattooed parts of wrists, as well as elsewhere on the body and found that on non-inked parts, the sensors gave uniformed readings.

However, 'on tattooed sections, sensor readings varied wildly depending on colours and shading,' Serenity Caldwell reported.

She said solid colours in darker inks such as black and red 'confused' the sensor the most, causing misreadings of up to 196 beats per minute (BPM), for example.

The average heart rate is 72 BPM.

Tests on lighter colours such as purple and yellow caused slightly elevated misreadings of 80BPM, compared to 69BPM on the wearer's non-tattooed wrist, but didn't interfere with skin contact detection, unlike with darker shades of ink.

The team was unable to reproduce problems reported on Twitter of light, but patterned tattoos creating errors, leading them to conclude that the design and type of tattoo makes a difference in how well the watch works.

Apple has yet to comment on 'tattoo-gate' but admits on its support page that 'many factors' can affect the performance of the Apple Watch heart rate monitor. 

'Skin perfusion is one,' it says.

'A fancy way of describing how much blood flows through your skin, skin perfusion varies significantly from person to person and can also be impacted by the environment. 

'If you're exercising in the cold, for example, the skin perfusion in your wrist may be too low for the heart rate sensor to get a reading.'

Apple said that motion is another factor. 'Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.' 

BUT APPLE WATCH'S SAPPHIRE GLASS SCREEN IS SCRATCH-PROOF 

The sapphire glass screen in the Apple Watch has remained scratch-free after being subjected to being rubbed with sandpaper, keyed and even hit with a hammer.

The test was carried out by Cardiff-based iPhone repairs specialist iPhoneFixed.co.uk, which used a sapphire screen from a 38mm Apple Watch for its experiment.

Two out of the three models of Apple Watches have sapphire glass screens - the Apple Watch and Apple Watch Edition.

The Apple watch is scratch tested from coins to power drills!

The glass is made from synthetic sapphire, which is created by applying heat and pressure to aluminium oxide powder, before the crystals are processed to remove impurities.

The tiny round disks created are ground into shape, and polished, to become glass.

Sapphire glass is said to be twice as tough as normal glass and almost as hard as a diamond.

To test the durability of the screen, it was rubbed against a stone wall and then with a coin, before being 'scratched' by a key.

The video capturing the attempts at destruction, shows no scratches were achieved to the screen.

It also withstood being rubbed with abrasive sandpaper at some force and hit by a hammer – albeit not very hard.

But most impressive, was the screen's ability to withstand an electric drill being dragged across its surface, without leaving a mark.

The test led iPhoneFixed to claim: 'Sapphire glass is pretty strong and virtually impossible to scratch'.