Capital Letters: Nairobi

My waist size is 32; the label said 36. They were the wrong trousers, but the hawker was having none of it.

"You are 32" he muttered sadly, looping his tape measure around my belt. Then, palming a few inches of tape and pulling it round me again, he added hopefully: "Or maybe 36?"

For many of his customers, at a roadside stall in Nairobi's commercial suburb of Westlands, that might have been a close enough fit to make do.

The trousers were on sale at a stall selling "mitumba" clothes - donations to charity shops in the west which are sold on to traders in Africa.

For many Kenyans, mitumba offers a means of dressing well for those who cannot afford new clothes.

The downside is that most of the clothes are those unlikely to find buyers in western charity shops; they often come in outdated styles or repulsive colours.

When I arrived in Nairobi, I expected this African metropolis to be filled with men and women dressed in loose, flowing robes and colourful headdresses. Instead, Kenyans seemed to prefer shapeless polyester slacks and mustard-yellow jumpers.

Some suggest the Kenyan fashion sense was warped by the experience of being colonised by men in knee-length khaki shorts and pith helmets, but others say the culprits lie on every street corner.

Mitumba clothes are everywhere. In shacks roofed with plastic bags and lined with mud and sticks, hawkers offer skirts and blouses from Esprit and Jigsaw.

Ralph Lauren shirts are slung over wooden poles, and Nike trainers hang in strings like onions at street stalls.

The clothes are undoubtedly cheap. Designer men's shirts sell for around £2 and skirts bearing British high street labels for less than £3.

And they are not all bad. At Nairobi dinner parties, smartly dressed Kenyan women will discreetly compare their mitumba purchases in the same way that British women appraise buys from the January sales.

But mitumba has blighted Kenya in other ways. As it has elsewhere in Africa, the flood of cheap clothes has swamped the textiles industry, which cannot compete.

The cotton farmers of western Kenya complain that they can no longer find domestic buyers for their products. Instead, African raw materials are exported to factories in Sri Lanka or China, before going on sale at a high street near you, then finding their way back to Africa's cities a decade later.

It is not just the clothing market that is dominated by cheap cast-offs. Kenya's roads are crammed with second-hand jalopies imported from Japan and the Gulf, while its poorer citizens live in shacks constructed from the jetsam of richer society.

Some Kenyans fear that wearing westerners' discarded clothes has a damaging psychological impact too.

The Kenyan government recently launched a search for a national dress. In a country where ethnic diversity is reflected in a kaleidoscope of fashion - from the scarlet cloaks of Masai warriors to the voluminous saris of Gujarati grocers - this is a vexed question.

But whatever design triumphs, mitumba will remain the unofficial national dress.