Wearing a fitness tracker during a marathon may add up to 11 miles to a runner’s race, a study has found.

Ahead of Sunday’s London Marathon, research suggested several trackers do not provide wearers with accurate information on the distance they have covered.

Consumer company Which? tested the reliability of 118 fitness trackers and smartwatches.

It discovered the Garmin Vivosmart 4 – which costs £119.99 – was the least reliable gadget tested. It added 10.8 miles to the marathon distance, meaning that by the time the device had clocked 26.2 miles, runners would have in fact run 37 miles.

Samsung’s Gear S2 was also found to miscalculate the distance according to the research, saying the wearer had reached marathon distance only after they had run 36.2 miles.

The Misfit Ray, Xiaomi Amazfit Bip, Fitbit Zip and Polar A370 were all also named as devices which carried runners past the 30-mile mark in testing.

Meanwhile the Huawei Watch 2 Sport watch – which is available to purchase for £179.99 – took seven miles off the correct distance, indicating runners had completed it after just 18.9 miles.

Of the several Apple watches tested, the Apple Watch Series 3 GPS – which retails between £279 and £379 – was the least reliable, saying the marathon distance target had been hit at 22.8 miles.

In order to test the fitness trackers and smartwatches, Which? used a calibrated treadmill to compare the accuracy of the gadgets when logging steps and when recording distance travelled.

Natalie Hitchins, head of home products and services at Which?, outlined the importance of ensuring fitness trackers are not “jeopardising” the finish times of marathon runners.

“Our tests have found a number of models from big-name brands that can’t be trusted when it comes to measuring distance, so before you buy, make sure you do your research to find a model that you can rely on,” Ms Hitchins said.

A Garmin spokesperson explained that the sports brand’s Vivosmart 4 watch may not have fared well in the study because it does not feature GPS.

“The Vivosmart 4 is an all-round smart fitness activity tracker used to monitor wellness, health and fitness for a range of activities,” the spokesperson said.

“As the Vivosmart 4 does not incorporate GPS, our recommendation for someone who is running long distances such as a marathon would be to choose a tracking device such as our Forerunner range, which is dedicated to running and incorporates GPS.”

A Huawei spokesperson said ”individual runner variances” may have affected the study’s findings.

“With regards to running indoors, as this particular test was carried out on a treadmill, the algorithm of Huawei Watch 2 Sport calculates the user’s stride length from the acceleration sensor data while running at different speeds.”

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