The greatest contribution that plastic bags have made to human society is their use as a toilet. In developing countries, the bags are commonly used as a repository for human faeces, where they end up hanging from trees. It is not pretty, and not particularly environmentally friendly, but it is better than the alternatives, of allowing detritus to make its way into drinking water supplies and thus spreading disease.

Still plastic bags are found polluting waterways and ending up in the sea, where they are a menace to marine life. Earlier this year, a whale was found to have died of plastic pollution, its guts clogged up with our packaging castoffs. The problem is so great that there is now a floating pool of rubbish in the Pacific, greater in extent than any other detectable man-made impact on the environment.

So when Nick Clegg, depute prime minister, announced a charge for plastic bags at the Liberal Democrat annual conference, there was cheering among delegates hungry for a new way to emphasise the party's commitment to the environment. The charge – if it comes about, and there are doubts as to how it will be implemented, and its efficacy as a result – should deter people from using the bags. And in the process, tackle a potent symbol of throw-away consumerism.

But plastic bags are only a small part of the problem. They account for only 0.03% of marine litter, according to the industry organisation Incpen.

The packaging that we all use, in day-to-day activities from buying food in supermarkets to our deliveries from online shopping centres, has a much greater – though less obvious – effect on pollution. A much greater percentage of non-biodegradable litter comes from food packaging such as the wrappers around food stuffs in supermarkets. Moves are afoot to cut that, supported by the retailers themselves, but there is still a long way to go.

Charges for plastic bags have already been introduced in parts of the UK, including Wales and Northern Ireland, so we already have an indication of how the policy could work in practice. Anna Beggs, from Northern Ireland, where the charge is already in force, told the Guardian: "I try to remember to bring my own bags so that I don't have to pay. If most people do that it will cut down on the plastic bag blight, especially in the countryside." The charge is 5p, compared with 25p in Ireland.

Charging for plastic bags demonstrably cuts down on their use. A Welsh Assembly official said: "Since we introduced our 5p carrier bag charge in October 2011, bag use in Wales has reduced by up to 96% in some retail sectors and over £4m worth of proceeds from the charge have been passed onto good causes, which include environmental charities such as Keep Wales Tidy, children's charities and cancer charities. Since the introduction of the charge, people in Wales have changed the way they shop. It has encouraged shoppers to stop unnecessarily accepting new bags every time they are at the till and checkouts in Wales are now full of people reusing their bags."

The charge is not technically a tax but is paid into a fund that goes to good causes.

Maggie Dunn, a Labour party activist, says that charging for the bags in England, as Clegg has suggested, is overdue. "I support this – it is unacceptable, how many bags we throw away. We need to think about the consequences – they are in the sea, they are harming nature." Her view is that people will accept the proposed charges, if they are introduced, but that they need to be higher to people from using the bags. She suggests 50p would be more effective.

Despite its reputation as the epitome of extravagant waste, packaging such as plastic films and paper wrappings for food, also play their part in environmental pollution. Companies and retailers that routinely rely on packaging point out that when food is spoiled for lack of preservative wrappings, the environmental cost is much greater than the impact of bags. In India, for example, and other developing countries, the UN has calculated that the spoiling of edible foods means that as little as half of the quantity produced makes it to market in an edible condition. The lack of cold storage facilities and poor refrigeration accounts for some of that, but the waste is one of the biggest factors in making it hard for the world to feed itself – an increasing problem in the context of a global population estimated to top 10bn by 2050, and the need to increase food production by more than half to cater to that rapidly growing need, according to the UN.

"People equate plastic with waste and that is understandable, but what people don't realise is that packaging has a job to do – ensuring that the product doesn't get overheated on the dock, or in the lorry, or to deliver the goods in a good condition," says Jane Bickerstaffe of Incpen.

Take a case in point - cucumber growers, who need to preserve their fast deteriorating food as soon as it is picked. "A cucumber wrapped in plastic needs only about 1.5 grams of plastic in its wrapper, but that extends the life of the product from about three days to at least 15 days, and when you look at the effort and environmental impact of growing a cucumber, the water and the fertiliser and all the rest, you can see we are preserving resources."

Bickerstaffe is alive to the impacts of plastic packaging, but she urges people to take a broader view than the rubbish that they fill their household bins with. "It is understandable that people do not think beyond their own experience. They take it for granted. But they don't realise that the vegetable wouldn't have got to the shop without plastic." Companies are also taking the lead in recycling plastics, reducing the amount of packaging they use – which also cuts their costs – and finding new materials that can be substituted for polymers. But Bickerstaffe admits: "I don't think we have the answers yet."

Big retailers are also taking measures to cut their packaging use overall. Sainsbury's was the first major UK retailer to offer milk in bags, reducing packaging by 75%, with the bags easy to recyclable at stores. The retailer says it has also achieved an estimated 14% reduction in packaging for plastic milk bottles across the range after adopting a new shape and style. Other products are also in for reduction: last year, the company cut the diameter of the inner cardboard tube on every one of its own-brand toilet rolls by 12mm, and that meant the number of delivery lorries required were reduced by the equivalent of 140,000 kg of CO2.

Being green is not quite as simple as cutting packaging, however. A further problem is that when companies seek to find alternatives to plastic, these are sometimes incompatible with current recycling techniques. Most local authorities in the UK cannot at present recycle plastic film, and when the new generation of biodegradable plastics are included in general plastic wastes, they can contaminate the waste and as a result render it unsuitable for current recycling technologies.

One local authority told the Guardian: "It's a nightmare because people think they are doing the right thing but if they put these new materials into their recycling bins, we can't help them. We are geared to one sort of packaging, and it's hard to re-engineer our systems to deal with another."

For volunteers on the cutting edge of plastic waste, the changes can't come soon enough. The Marine Conservation Society organises clean-ups around the UK's coast on a regular basis, relying on volunteers to give up their weekends to reduce the amount of litter that is both an eyesore and a severe threat to marine life. For those manning the beaches, the biggest eyesore is one that is created by well-meaning members of the public - those who use plastic bags as a toilet, not for themselves but for their pets. Dog mess carefully scooped into plastic bags and deposited, equally carefully, on the footpaths, in parks, on country trails, on beaches is now the biggest waste issue in the UK, according to the MCS.

"People who clean up after their pets by shovelling the poo into plastic bags may think they are doing the right thing, but unless they then put the bags into a bin, they are doing worse than leaving it where it landed."

The plastic bags are a blight, and they prevent the faeces from degrading or being washed away. So what may seem to be a public-spirited act is creating litter and environmental damage.

Laura Foster, pollution programme manager at MCS, said: "Plastic is extremely resistant to biodegradation, and degrades into increasingly smaller particles – estimates for plastic degradation at sea range from hundreds to thousands of years.

"Last year plastic was the number one litter item found on our beaches A survey done of Northern fulmars found that 95% had plastic in their stomachs. In relation to discarded dog poo bags – we encourage dog walkers to bag it and bin it."