Harkila Heat Long Johns are long under pants with belt ï heating systems made from 80% nylon and 20% spandex, making them quick-drying, stretchy and hardwearing. They have a modern cut, fitting relatively snugly to the body. In the parts of the lower body most vulnerable to cold – in the lumbar region and just above the knees – there are heating membranes sewn in.

The membranes can be set to 35C, 43C or 50C using the push button in the waistband. Unlike older heating technologies, the heating membranes are flexible enough to be folded up and packed away with the garment they are sewn into. The long johns can also be safely machine-washed. In the waistband is a long, fabric-covered cord with a waterproof USB plug, for connecting a power bank of the desired capacity. The cord is long enough so you can keep the power bank out of the way in your trouser pocket, for example. With a power bank of 10,000mAh, the Harkila Heat Long Johns can output their highest temperature of 50C for about 7 hours at an outdoor temperature of 0C. To save energy, the heating membranes automatically go into standby once the garment has reached the set temperature.

They then turn back on automatically if the temperature falls 1-2 degrees below the set level. The result is that the Harkila Heat Long Johns are heat-regulating, because the heating function automatically turns off if you get hot from intense exertion and turns back on when you are sitting or standing still.

These cosy long johns are supplied with a drybag to pack them away in and make a great gift.