Meet Montane’s top-performing thermal combination. The 140g top is light and breathable but still provides surprising warmth thanks to a mix of Merino wool and Primaloft insulation (a brand of synthetic insulation that stays warm even when wet), while a covered zip neck provides extra ventilation. This was one of the most flattering tops we tried out, and looks good worn with jeans as well as hiking trousers. The thermal leggings are a versatile investment – they’re slim-fitting and warm enough to work as base layers but also excel when worn alone as trail running or hiking tights, with handy zipped bottoms for pulling on and off over hiking boots plus a high snug waistband to keep the tights in place while you’re exercising. Added touches include flatlocked seams and multiple handy pockets. Sold separately.  

All of Picture’s outdoor-ready clothing is organic and ethically produced, making the brand a great one-stop shop for cold weather gear if you care about the eco credentials of your clothing. This set of base layers have a soft cotton-y feel next to the skin, but are actually made of recycled polyester. The top has a nice boxy cut and handy thumbholes, and the warm, versatile leggings stood out on test – they were some of the most comfortable thermal bottoms we tested, with a wide waistband that stays put when you’re getting active. The cheerful peony print makes this base layer set versatile enough to wear anywhere.

Bamboo is one of the newest but also one of the best materials to hit the activewear market – as well as being a sustainable, pesticide-free crop, it naturally lends itself to base layer use, keeping you cool when you’re moving but trapping in heat when you’re staying still. It’s also very soft, making it ideal for sensitive or allergy-prone skin. We love this stretchy top, which is form-fitting enough to trap in heat but is still flexible and breathable enough to wear for fast-paced sports such as running. Stiches are flatlocked, creating a top you’ll barely notice you’re wearing. If you like something thicker, we also love BAM’s quarter-zip sweat, £55, which works as either a base layer or a mid layer.

If you like to feel as unencumbered as possible when you’re layering up for the cold, this “barely-there” top is for you. One of the stretchiest tops we tested, it’s cut long, sitting on the hips and staying in place when you’re moving. Despite feeling light to wear, it retains heat well, and Columbia’s own moisture-wicking technology effectively does that. It’s a pity the zip on the high neck isn’t covered, so it can be a tad scratchy when fully zipped up.

Who needs bells and whistles on their base layers when Finisterre makes such simple, smart thermal sets? The top and leggings are made with organic cotton and bamboo, and have flatlocked seams and a small amount of stretch for great comfort – besides that, they are pleasingly minimalist, and the top’s neutral looks work well for casual use as well as for winter sports. Finisterre takes its eco credentials seriously, and as well as using bamboo to create the set, the plastic packaging the base layers come in is water soluble and recyclable. Although sold separately, £90 for both items is a decent price point, too.

Slip on these base layer and the first thing you’ll notice is how deliciously soft its 100 per cent  Merino wool make-up feels. The top has flatlock stitching as well as all of Merino wool’s great qualities – it’s highly breathable, wicks away sweat and is odour resistant. Keela offers a good size range, going up to a size 20, and we love the jewel purple tone, which makes a nice change from boring black base layers. It also scores highly for value for money - we reckon £50 is good value for a 100 per cent Merino top, which can often cost twice that.

Smartwool, as the name suggests, specialise in great Merino wool layers, and with 50 styles to choose from for women you’re likely to find one you love. Our top pick is Goldilocks-lik and is just right – it feels lightweight but instantly warms your core once you pop it on. It was also the comfiest of all the base layer tops we reviewed, with a lovely knit-like feel that also looks rather smart. It has a longer cut and is very figure hugging, which is great for layering, but if you prefer the looks of a boxier cut we’d suggest ordering a size up. Bright patterned versions and matching leggings are also available.

Haglofs was founded in Sweden, so it’s not a surprise it has an eye for cold weather gear. The no-nonsense 90 percent Merino actives range of base layers is designed with snowsports and hiking in mind, and the top and long johns are form-fitting and easy to layer, with well-placed flatlock seams that mean you’ll barely feel your thermals even as you walk, ski or board. The top works as a wardrobe staple, but the leggings are a little thin to stick on for casual use or a yoga class. The top and bottoms are pricey at £90 each, but we reckon they’re worth it for serious sports lovers, considering they pack a Merino wool punch. Sold separately. 

Go bold with your base layers with Dare2B’s cheerful divulge top, designed in a bright floral print. The stretchy polyester fabric feels silky against the skin, and despite a looser cut the top still fits reasonably snugly thanks to stretchy side panels, which also allows you to pop a vest or t-shirt underneath.The half zip is flattering and allows for extra ventilation, and is covered to stop the zip irritating your neck. The base layers aren’t the warmest, and might not suit more ambitious adventures, but they start at just £10.45, making them good choices if you don’t want to spend a lot more.

These are our snug-as-anything top picks for dedicated snowsports lovers. At 225g they’re a nice balance of warmth and weight, and as well as an outer layer of 100 oer cent Merino wool there’s an inner layer of material, which is designed to wick away sweat and trap in body heat, regulating your temperature whether you’re skiing hard or just sat on a lift. The hoodie adds an extra level of warmth for winter, saving the need for a neck warmer and fitting underneath a helmet.  

We always rate Icebreaker’s great quality, sustainably sourced Merino layers on test, and they’ve stepped up the ethical offerings with these layers. This top is dyed with plant pigments, an eco-friendly process that uses less water than synthetic dying as well as giving a pleasingly earthy tie dye look. The 200 weight is a good warmth to weight ratio for most outdoor adventure. Icebreaker’s Merino layers are some of the most expensive on sale, but also some of the best, and will last you for years without losing their shape or their warming abilities.

If your activewear motto is ‘the brighter the better, then Roxy’s daybreak set is the one for you,which is back for this season in eye-catching floral, graphic and mountain prints. Designed with skiers and snowboarders in mind, these sleek thermals have flatlocked seams and a comfortable lightweight fit that layers up nicely under snowsports clothing and hiking kit. We love the longer length of the top and the slim stretchy fit of the leggings, which would also work well for yoga and gym classes. Sold seperately.