Instead of throwing stained, ripped and misshapen clothing in the bin, Britons are being asked to stick the dregs of their wardrobe in the post in a trial aimed at tackling the “staggering” quantity of textiles sent to landfill or incinerated each year.

A third of consumers do not know what to do with tops, dresses and trousers that can no longer be worn, figures show, with a similar number admitting to putting such items in their household waste bin.

Now unwearable clothes from any label can be returned in a prepaid postal donation bag left with a courier as part of the experimental tie-up between Marks & Spencer and Oxfam, which runs alongside its existing scheme for wearable items.

Katharine Beacham, M&S’s head of materials, sustainability and packaging, said the scheme made it possible for someone to clear out all their unloved clothing in one go. “Whether it is wearable or unwearable, we want it all,” she said.

M&S and the charity have for a number of years been working together on the “shwopping” initiative, in which customers drop off old clothing in exchange for loyalty card perks.

However, the postal scheme, which is being paid for out of a new £1m accelerator fund linked to the retailer’s ethical project Plan A, is part of a wider push to find ways to reduce textile waste. Research suggests the UK’s wardrobes contain 1.6bn items of unworn clothing.

The bags can be ordered on the Oxfam website, and individuals are asked to enclose unwearable items in a separate sack. With a fifth of consumers telling M&S they did not know how to discriminate on wearability, the anti-waste charity Wrap stresses that “wearable” clothing is clean, dry, in good condition and ready to be worn. “Unwearable” items are damaged in some way, for instance torn, stained, faded, or stretched.

Consumers can also use the service to donate preloved soft furnishings such as bed linen, towels, cushions, tablecloths and tea towels. However, the M&S in-store “shwopping” scheme continues to be for wearable, hand-me-down quality clothing only. Individuals are asked not to include soiled or contaminated clothing as it cannot be recycled.

Still wearable donations will be sold through Oxfam’s stores and website, while the “unwearables” will be responsibly recycled by a UK Fashion and Textile Association (UKFT) project. It is working on a blueprint for an advanced textile sorting and pre-processing (ATSP) centre that would be capable of turning clothing unsuitable for resale into new garments, resulting in a completely circular system.

Adam Mansell, the chief executive of UKFT, said urgent action was needed to tackle the “staggering amount” of textile waste that ended up in landfill or incinerated each year. “We’re aiming to encourage people to separate their items so that in future, worn-out clothing can make its way to an automated sorting facility and then be recycled into new textiles and garments here in the UK,” he said.