Our woodlands are much loved. They provide some of the biggest opportunities for outdoor recreation in the UK. Every year, millions of walkers, cyclists and horse-riders enjoy them. For many, they are unrivalled as an accessible way to get fresh air and enjoy nature, well-served with accessible trails and visitor centres. It's not surprising that government plans to sell off these national treasures are generating widespread dismay and alarm.

The government seems to be starting to realise how unpopular plans to sell off the forests could be. In the last few days, Caroline Spelman, the environment secretary, has gone on a charm offensive, full of warm words for our "precious natural assets". She reassures us that whoever owns the forests, laws to protect biodiversity and public access would remain. That's true, but to claim it means we shouldn't worry about a sell-off is deeply misleading.

For a start, existing laws won't preserve most of what we value about our publicly owned forests. Take the Countryside and Rights of Way Act. It's an important piece of legislation, but it won't safeguard for future generations the kind of access to forests that we take for granted. New owners wouldn't be allowed to put up a "keep out" sign. But they wouldn't be required to maintain the tracks, signage, or visitor centres that make visiting the forests an easy pursuit for millions.

The Countryside and Rights of Way Act will also do nothing to protect access for those who enjoy our forests on mountain bikes or on horseback rather than on foot, or who rely on wheelchair or buggy-friendly tracks. Almost every single purpose-built, single-track mountain-bike trail in England is in a publicly owned forest. There are none in privately owned forests. If I were a mountain biker, I'd be fearing for the future of my sport.

Our publicly owned forests are havens for wildlife in increasingly stressed English ecosystems. The Forestry Commission currently manages all our forests to FSC standards, well above and beyond the standards required by law. Higher standards of stewardship mean that even those publicly owned woodlands that deliver substantial profits for the taxpayer through timber are also havens for endangered animals such as the red squirrel. Some privately owned woods also meet the standards, but others do not. If the forests are sold off, nothing in the law would require future owners to maintain these same high standards of stewardship.

The bigger point here is that as long as the public owns these "precious natural assets", we have a say in decisions about their future. We can ensure that future generations have the option to guarantee the forests as places of enjoyment and biodiversity as well as timber production. We can ensure that future governments grappling climate change and the ecosystem stress it creates can use the forests to mitigate its impacts. Once they are sold, we've lost our say for ever. Future generations would have to rely on the goodwill of the future private owners.

So why sell off the forests? It's hard to see this being a big fundraiser for the treasury. Our national forests are a bargain to the taxpayer – managing the entire estate costs each of us a little less than 30p a year. The one-off cash dividend is likely to be small, unless you're willing to really give developers free rein to replace ancient woodland with Center Parcs-style leisure complexes and golf courses. The more conditions you place on future owners to guarantee public access or safeguard biodiversity, the less you are likely to raise money.

I suspect that the government's enthusiasm for the selling of the forests may be rather more ideological than that. Our forests are a huge publicly owned asset. Publicly owned assets, even much-loved ones delivering clear social and environmental benefits, have never been something Conservatives are particularly keen on. This is a classic case of "selling off the family silver", and one which we should oppose.

The key question, perhaps, is does privatising our woodlands take us closer or further away from the kind of society we'd like to live in? Many of us don't have to think for too long before answering that question. It is not surprising to me that the 38 Degrees campaign against selling off the forests is their fastest-growing to date. It is also one of the most important – I'd urge you to support it.