Cold weather survival: How Nato troops secure food, water and fire in harshest winter wilderness

Nato troops must be able to defend the alliance in any conditions, so UK soldiers from Nato's multinational battlegroup in Estonia, bordering Russia, have been learning vital winter wilderness survival skills.

The Cold Weather Operator's Course, which takes place each year in Estonia, involves soldiers spending two nights in a forest, learning how to build camps, make fires and find food and water.

Colour Sergeant Sykes, from the Royal Marines, is chief instructor on the course and an expert in how to sustain yourself in a cold weather environment.

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CSgt Sykes explains that survival is "highly dependent on the environment that you're in", but says "as a general rule of thumb, three days without water and three weeks without food, that's probably as long as you're looking to last without those vital necessities for life."

For enduring a sub-zero Estonian winter, the priority is initially heat and shelter, followed by water and then food.

The size of your shelter will depend on the number of people to accommodate, but "the basic material you need to collect are logs to build a frame and then also plenty of brushwood to retain that heat".

Tinder is needed to start a fire and CSgt Sykes recommends birch bark which "works particularly well, even when wet".

You then progress up to kindling, he says, with "small sticks no thicker than a finger" and finally can start adding larger logs as fuel.

To help you operate in such an extreme environment, the chief instructor says flint and steel are best "to start a fire that will 100% work all the time when wet".

Other essential items for cutting wood are "some form of small folding saw and also a small knife with which to process firewood".

Water is easy to come by but both snow and running water should be boiled before drinking to ensure they are safe.

For food, CSgt Sykes suggests "some sort of snare device to catch the small animal", which you will need to "process and cook so it's safe to eat".

Prior to venturing out into the wilderness, he also recommends plenty of research, so you know "what flora is actually edible out here in the wild".