Consumer watchdog Which? has warned that most tea-bags being sold today are only 70-80% biodegradable. They say that the outer net part persists after the tea and cotton has rotted away, due to heat-resistant polypropylene that is used to seal the bags.

The government waste body WRAP however, says this doesn't matter. Lynne Gunn, Wrap's home composting expert, said: "Our advice remains that teabags are suitable for composting. If the bags are still visible when you want to use the compost, they can be sieved out or picked off the surface of the soil. You can also speed up the composting process by ripping open the bags."

Harriet Kopinska, speaking for Garden Organic, said: "Composting is still the better environmental option. But even better, use loose tea."

Teabags account for 96% of the 165m cups of tea drunk every day in the UK according to the UK Tea Council, which also says that we gulp down 165 million cups daily or 60.2 billion per year. Assuming one teabag per cup that is about 30,000 kilos per year to compost.

Which? said the recyclability of teabags did not seem to be high on manufacturers' agendas - it found only one polypropylene-free brand: Jacksons of Piccadilly.

But other fully biodegradable ones include those that are stitched rather than sealed and Japanese-style pyramids such as "tea temples" manufactured by teapigs.co.uk, which contain whole leaves and has recently changed its bags from nylon mesh to a 100% compostable corn starch.

Other manufacturers have responded to the news. PG Tips said,  "Our teabags are about 80% paper fibre, which is fully compostable along with the tea leaves contained in the bag. The remaining packaging includes a small amount of plastic which is not fully biodegradable."

Teadirect said, "Our teabags are 70% compostable". Its sustainability manager Whitney Kakos said the use of polypropylene was "an industry-wide practice".

Clipper Teas - who market fair trade brands - said, "Our teabags are composed of vegetable and wood fibres and are sealed with a low level of polypropylene. Our teabags are suitable for inclusion in compost heaps where the fibres will break down. Any remaining fibres will be small and can be dispersed in the soil."