Owners of the coveted watch have found the timepiece's sensors malfunction when worn on tattooed wrists.

Dark, vibrant ink seems to cause the watch’s heart rate monitor to lose connection and give inaccurate readings.

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

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.

Apple has yet to comment on the problem, despite there being speculation that the apparent fault could also stop some people using Apple Pay.

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.

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.' 

(dailymail.co.uk)