Britain was the principal slaving nation of the modern world. In The Empire Pays Back, a documentary broadcast by Channel 4 on Monday, Robert Beckford called on the British to take stock of this past. Why, he asked, had Britain made no apology for African slavery, as it had done for the Irish potato famine? Why was there no substantial public monument of national contrition equivalent to Berlin's Holocaust Museum? Why, most crucially, was there no recognition of how wealth extracted from Africa and Africans made possible the vigour and prosperity of modern Britain? Was there not a case for Britain to pay reparations to the descendants of African slaves?

These are timely questions in a summer in which Blair and Bush, their hands still wet with Iraqi blood, sought to rebrand themselves as the saviours of Africa. The G8's debt-forgiveness initiative was spun successfully as an act of western altruism. The generous Massas never bothered to explain that, in order to benefit, governments must agree to "conditions", which included allowing profit-making companies to take over public services. This was no gift; it was what the merchant bankers would call a "debt-for-equity swap", the equity here being national sovereignty. The sweetest bit of the deal was that the money owed, already more than repaid in interest, had mostly gone to buy industrial imports from the west and Japan, and oil from nations who bank their profits in London and New York. Only in a bookkeeping sense had it ever left the rich world. No one considered that Africa's debt was trivial compared to what the west really owes Africa.

Beckford's experts estimated Britain's debt to Africans in the continent and diaspora to be in the trillions of pounds. While this was a useful benchmark, its basis was mistaken. Not because it was excessive, but because the real debt is incalculable. For without Africa and its Caribbean plantation extensions, the modern world as we know it would not exist.

Profits from slave trading and from sugar, coffee, cotton and tobacco are only a small part of the story. What mattered was how the pull and push from these industries transformed western Europe's economies. English banking, insurance, shipbuilding, wool and cotton manufacture, copper and iron smelting, and the cities of Bristol, Liverpool and Glasgow, multiplied in response to the direct and indirect stimulus of the slave plantations.

Joseph Inikori's masterful book, Africans and the Industrial Revolution in England, shows how African consumers, free and enslaved, nurtured Britain's infant manufacturing industry. As Malachy Postlethwayt, the political economist, candidly put it in 1745: "British trade is a magnificent superstructure of American commerce and naval power on an African foundation."

In The Great Divergence, Kenneth Pomeranz asked why Europe, rather than China, made the breakthrough first into a modern industrial economy. To his two answers - abundant coal and New World colonies - he should have added access to west Africa. For the colonial Americas were more Africa's creation than Europe's: before 1800, far more Africans than Europeans crossed the Atlantic. New World slaves were vital too, strangely enough, for European trade in the east. For merchants needed precious metals to buy Asian luxuries, returning home with profits in the form of textiles; only through exchanging these cloths in Africa for slaves to be sold in the New World could Europe obtain new gold and silver to keep the system moving. East Indian companies led ultimately to Europe's domination of Asia and its 19th-century humiliation of China.

Africa not only underpinned Europe's earlier development. Its palm oil, petroleum, copper, chromium, platinum and in particular gold were and are crucial to the later world economy. Only South America, at the zenith of its silver mines, outranks Africa's contribution to the growth of the global bullion supply.

The guinea coin paid homage in its name to the west African origins of one flood of gold. By this standard, the British pound since 1880 should have been rechristened the rand, for Britain's prosperity and its currency stability depended on South Africa's mines. I would wager that a large share of that gold in the IMF's vaults which was supposed to pay for Africa's debt relief had originally been stolen from that continent.

There are many who like to blame Africa's weak governments and economies, famines and disease on its post-1960 leadership. But the fragility of contemporary Africa is a direct consequence of two centuries of slaving, followed by another of colonial despotism. Nor was "decolonisation" all it seemed: both Britain and France attempted to corrupt the whole project of political sovereignty.

It is remarkable that none of those in Britain who talk about African dictatorship and kleptocracy seem aware that Idi Amin came to power in Uganda through British covert action, and that Nigeria's generals were supported and manipulated from 1960 onwards in support of Britain's oil interests. It is amusing, too, to find the Telegraph and the Daily Mail - which just a generation ago supported Ian Smith's Rhodesia and South African apartheid - now so concerned about human rights in Zimbabwe. The tragedy of Mugabe and others is that they learned too well from the British how to govern without real popular consent, and how to make the law serve ruthless private interest. The real appetite of the west for democracy in Africa is less than it seems. We talk about the Congo tragedy without mentioning that it was a British statesman, Alec Douglas-Home, who agreed with the US president in 1960 that Patrice Lumumba, its elected leader, needed to "fall into a river of crocodiles".

African slavery and colonialism are not ancient or foreign history; the world they made is around us in Britain. It is not merely in economic terms that Africa underpins a modern experience of (white) British privilege. Had Africa's signature not been visible on the body of the Brazilian Jean Charles de Menezes, would he have been gunned down on a tube at Stockwell? The slight kink of the hair, his pale beige skin, broadcast something misread by police as foreign danger. In that sense, his shooting was the twin of the axe murder of Anthony Walker in Liverpool, and of the more than 100 deaths of black people in mysterious circumstances while in police, prison or hospital custody since 1969.

This universe of risk, part of the black experience, is the afterlife of slavery. The reverse of the medal is what WEB DuBois called the "wage of whiteness", the world of safety, trustworthiness, welcome that those with pale skins take for granted. The psychology of racism operates even among those who believe in human equality, shaping unequal outcomes in education, employment, criminal justice. By its light, such all-white clubs as the G8 continue to meet in comfort.

Early this year, Gordon Brown told journalists in Mozambique that Britain should stop apologising for colonialism. The truth is, though, that Britain has never even faced up to the dark side of its imperial history, let alone begun to apologise.

Dr Richard Drayton is a senior lecturer in imperial and extra-European history since 1500 at Cambridge University. His book The Caribbean and the Making of the Modern World will be published in 2006.

RHDrayton@yahoo.co.uk