The small stockroom of St Raphael's Hospice charity shop in Cheam, south London, is rammed with donations. "On average, we get 30 to 50 bags a day," says Caroline Worley, the commercial manager, "but it's not unusual to receive over 100." This might sound like a good problem to have, but the sheer volume of stuff is the shop's biggest challenge. "Sometimes, there's only [enough] floor space to tiptoe," says 48-year-old store manager, Colin French.

Rummaging through the donations, it's clear that much is unsaleable: a smelly, soiled bathrobe, tatty trainers, heavily bobbled jumpers, old socks, an empty Swarovski jewellery case. "Absolutely typical," says Worley. They're also overwhelmed with deluges of low-quality fashion – a shift in donation habits that accelerated during Covid, because, says Worley, all that time at home enabled people to sell their best stuff on Vinted.

The experience here echoes that of author Tansy Hoskins, whose latest book, Charity Shop World, paints a picture of a charity retail sector in crisis. She spent 16 months working in a London charity shop, sorting through "daily avalanches" of clothing, which swamped "all the other categories put together".

Everyone Hoskins interviewed said similar: "quality is the big change marker – it's gone through the floor. You're just fighting against the waste." The problem is nationwide. Since the book came out, she's "been contacted by managers of charity shops around the country – for example, Nottingham, Accrington, Newcastle – all saying that most of what they're getting is waste."

It is, of course, our waste. According to Jose Baladron of Traid, which has 12 shops across London, shopping for new clothes is "through the roof – the UK is the highest consumer of clothing in Europe by far". "That obviously translates to more donations, which is far too much to process in small shops." Hoskins routinely found that ultra-fast-fashion donations would "still have tags on – it was clear people had received their online purchases and gone, 'What the hell is this?' It wasn't even wearable so they'd just donate it."

On average, charity shops only manage to sell about 25% of clothing donations, according to Baladron (a figure replicated by a University of Southampton study). "So they rely heavily on the ragman" to shift the rest. Yet the price of rag has "plummeted", according to Merryn Haines-Gadd of the environmental action charity Wrap, "due to the saturation of low-quality fast fashion". Ten years ago, a charity might have been paid 50-60p per kilo. "Now you're lucky to get 10p," says Baladron. Plus there's only so much the ragman can take.

For anything they can't take, charity shops have to pay to have it collected as waste. At St Raphael's, some of that has included, according to Worley, saucepans with food still in the bottom, babies' soiled sleep-suits and used sanitary products. "[Staff] can get a bit worn down when people are donating stuff that isn't saleable," says Libby Gordon, CEO of Fara, which has 39 charity shops in London. "Plus there's a time cost for us to process it all."

Factor in rising utility costs and national insurance payments, and "charity shops are in a perfect storm", says Hoskins. In June, the British Heart Foundation announced plans to close 150 of its 651 stores; its latest accounts indicate that its retail arm lost £3.3m last year. Cancer Research is part-way through a 190-shop closure programme, and rumours are swirling that up to 100 Oxfam shops might close (which the charity has denied).

Some are fighting back. Fara's new in-store poster campaign advises would-be donators: "If you'd be happy to give it to a friend, we'd be delighted to receive it." Items should be "clean, complete, and in good condition", states their online guide.

But just as people "wish cycle" by chucking unrecyclables into the recycling, so do people "wish give". I certainly struggle to accept that something with life left in it, however worn, can't be useful to someone. This mentality, however, creates unpleasant work and costs for charities – even more so when people leave donations outside a closed charity shop (technically, it's fly-tipping, and charity shops do often check their CCTV, record numberplates and report such crimes). These bags are often soiled by foxes or dogs, ripped open and/or rained on. At St Raphael's, staff have found soiled mattresses, ripped sofas and even an old toilet dumped outside: "It leaves us with a big problem," says Worley.

Disposal at scale is an expensive business. St Raphael's, which as a charity costs around £7m a year to run, and supports more than 1,000 patients a year with free end-of-life care, ends up paying £30,000 a year to dispose of dud donations. That could fund a month of around-the-clock specialist care for an inpatient; their 13-bed inpatient unit can currently only afford to keep 10 spots open. It's a similar story for other charities. "The public's generosity is how we support our work in Romania," says Gordon. "It's how the British Heart Foundation funds their research; it's how Mind supports their mental-health work."

French knows first-hand the difference money earned by charity shops can have. Last year his sister died, and in her final three weeks, received incredible care from Princess Alice Hospice in Esher, Surrey – as did the whole family. "My parents (still) go back every single week [for bereavement support] and don't have to pay anything," says French. "It's not until those moments that you truly realise that this is vital for our communities," he says.

The work a charity does can often feel a long way away when faced with a wardrobe in need of a clear-out. But, says Hoskins, try to "connect with why you like a charity, why it's important – whether it's the Red Cross or Cancer Research or an animal charity – and donate responsibly to them … It would be a complete tragedy if our charity shops vanished."

How can you make your good intentions go further?

Resist virgin fast fashion

The fast-fashion business model is about making clothes quickly and cheaply to churn out trends. It's bad news for our wardrobes, our purses and the charity shops that we pass them on to.

'Donate like you're being reviewed'

"Wash your clothes first, do simple mending tasks, or get it fixed at a free Repair Cafe," says Hoskins. There's a perception, she adds, that "charity shops have laundries – they don't. The number one reason we'd chuck something in the rag bag was because it was dirty."

Reframe how you view charity shops

Think of them not as convenient dumping grounds, but as places that deserve a little more thought. Plus, Hoskins says, "if you can afford it, donate your best bits instead of selling them on Vinted."

Research what to do with rag

Don't bin those odd socks, greyed bras, or stained items, as many of us do – according to Wrap, almost half of all used textiles ending up in the bin. There are specialist recycling programmes for …

For new pants and "gently worn, laundered" bras, try smallsforall, which distributes them to those in need; Bravissimo takes bras from any brand in any condition, plus swimwear, for redistribution or downcycling (in-store only). For odd or holey socks, the London Sock Company runs a "sock amnesty", recycling them into fingerless gloves for the charity Give Your Best (free UK postage).

Remill will recycle 100% cotton items (excluding bedlinen, denim and underwear) into new clothing, or where necessary, downcycle into cloths and insulation (free UK postage); plus you'll receive a voucher for its recycled-cotton clothing.

The Cashmere Centre will recycle your used, pure cashmere into knits, and give you a £10 voucher.

Sports shoes
JogOn, on a mission to save 1m sports shoes from landfill, takes trainers, football boots etc in any condition, redistributing the usable ones to those in need (its "save" rate is 90%), and sustainably disposing of truly end-of-life ones (postage costs £2 for up to 30 pairs).

General rag
Ease the processing burden on charity shops by taking end-of-life textiles to your local textile recycling bank (check Recycle Now's bin locator – you can also search which bins accept specific items; some take shoes). Ideally put your rag in a labelled bag to simplify sorting, tie pairs of shoes together, and if the bank is full, don't dump your stuff – it will most likely be binned.