Every year, thousands of us across the UK donate our used clothing to charity - many in the belief that it will be given to those in need or sold in High Street charity shops to raise funds. But a new book has revealed that most of what we hand over actually ends up getting shipped abroad - part of a £2.8bn ($4.3bn) second-hand garment trade that spans the globe. We investigate the journey of our cast-offs and begin to follow one set of garments from donation to their eventual destination.

Media playback is unsupported on your device Media caption How charity clothing donations end up traded abroad.

UK consumers ditch more than a million tonnes of clothing every year.

The Western world's growing desire for fast, disposable fashion, fuelled by the ready supply of cheap goods manufactured in China and elsewhere, means we are consuming and then disposing of an ever greater quantity of garments.

There's a moment of magic where a gift turns into a commodity Dr Andrew Brooks, King's College London

And, encouraged by charities and recycling companies, we are handing more and more of these old clothes over - via shops, collection bags or clothing banks - for reuse by new owners.

Almost half of the garments we now throw out end up going to a new home rather than ending up in landfill or at an incineration plant, estimates the Waste & Resources Action Programme (Wrap), a UK government and EU-backed agency tasked with reducing waste.

Few would dispute that diverting clothing away from landfill and giving it a new life is a good thing.

But Dr Andrew Brooks, lecturer in development geography at King's College London, argues in his book Clothing Poverty that many donors don't realise that the majority of the cast-offs they hand over to charity will be traded abroad for profit.

"The way most people encounter the second-hand clothing trade is their High Street second-hand store. I think there is a common presumption amongst the general public that if they give something to charity it's most likely to be sold in one of these shops, " he says.

"And while many garments are sold in these shops, the demand is relatively low compared to the supply, and far more get exported overseas."

Wrap estimates that more than 70% of all UK reused clothing heads overseas - joining a global second-hand trade in which billions of old garments are bought and sold around the world every year.

According to the latest available UN figures, the UK is the second largest used clothing exporter after the US. It exported more than £380m ($600m), or 351,000 tonnes, worth of our discarded fashion overseas in 2013. Top destinations were Poland, Ghana, Pakistan and Ukraine.

The US's key trade partners are Canada, Chile, Guatemala and India.

Image caption Source: The United Nations Comtrade Database - latest available full figures from 2013

So, how do our shirts, trousers, jeans and dresses end up thousands of miles away in a Polish thrift store or street market in Ghana?

The international journey of our cast-offs begins when charities sell on the clothing that cannot be sold in the UK's 10,000 charity shops.

The UK Charity Retail Association says 90% of garments handed over directly to shops "end up on the rack in store".

However, according to Brooks, as little as 10-30% of what is given to UK charities overall actually ends up being sold over the counter. It is a similar proportion in the US and Canada.

What isn't bought in shops is, more often than not, Brooks says, sold to textile merchants, who then sort, grade and export the surplus garments - converting what began as donations into tradable goods.

"There's a moment of magic where a gift turns into a commodity," says Brooks. "Like many used items, on the surface second-hand clothes may appear to have very little value, but through processes of sorting and transporting - turning disorderly objects into an ordered commodity - they are reproduced as retailable assets."

One of those businesses transforming our cast-offs is London-based family firm LMB Textile Recycling.

Director Ross Barry and his staff collect the contents of clothing banks - sometimes on behalf of charities - and check them by hand for quality. Those that pass the test are baled together and exported to the company's regular customers in Eastern Europe and Africa, where the garments are highly valued.

"They get cheap, affordable clothing that works for them, that's fashionable and that lasts a long time," he says.

Testing the system: Tracking six items of clothing

The onward journey of our clothing depends very much on what we have donated - different garments tend to end up in different places.

For example, Andrew Brooks's research found that white dress-shirts regularly headed to Pakistan, where there was a great demand among lawyers - warm coats often travelled to Eastern Europe, and short-sleeved tops and shorts, perhaps predictably, ended up in Africa.

We have handed over six items or groups of clothing, pictured above, and have started tracking them via GPS. We will report the results if and when they have reached their destinations.