David Cameron's ancestors were among the wealthy families who received generous reparation payments that would be worth millions of pounds in today's money. The true scale of Britain's involvement in the slave trade has been laid bare in documents revealing how the country's wealthiest families received the modern equivalent of billions of pounds in compensation after slavery was abolished.

The previously unseen records show exactly who received what in payouts from the Government when slave ownership was abolished by Britain – much to the potential embarrassment of their descendants. Dr Nick Draper from University College London, who has studied the compensation papers, says as many as one-fifth of wealthy Victorian Britons derived all or part of their fortunes from the slave economy.

As a result, there are now wealthy families all around the UK still indirectly enjoying the proceeds of slavery where it has been passed on to them. Dr Draper said: "There was a feeding frenzy around the compensation." A John Austin, for instance, owned 415 slaves, and got compensation of £20,511, a sum worth nearly £17m today. And there were many who received far more.

The British government paid out £20m to compensate some 3,000 families that owned slaves for the loss of their "property" when slave-ownership was abolished in Britain's colonies in 1833. This figure represented a staggering 40 per cent of the Treasury's annual spending budget and, in today's terms, calculated as wage values, equates to around £16.5bn.

A total of £10m went to slave-owning families in the Caribbean and Africa, while the other half went to absentee owners living in Britain. The biggest single payout went to James Blair (no relation to Orwell), an MP who had homes in Marylebone, central London, and Scotland. He was awarded £83,530, the equivalent of £65m today, for 1,598 slaves he owned on the plantation he had inherited in British Guyana.

But this amount was dwarfed by the amount paid to John Gladstone, the father of 19th-century prime minister William Gladstone. He received £106,769 (modern equivalent £83m) for the 2,508 slaves he owned across nine plantations. His son, who served as prime minister four times during his 60-year career, was heavily involved in his father's claim.

Mr Cameron, too, is revealed to have slave owners in his family background on his father's side. The compensation records show that General Sir James Duff, an army officer and MP for Banffshire in Scotland during the late 1700s, was Mr Cameron's first cousin six times removed. Sir James, who was the son of one of Mr Cameron's great-grand-uncle's, the second Earl of Fife, was awarded £4,101, equal to more than £3m today, to compensate him for the 202 slaves he forfeited on the Grange Sugar Estate in Jamaica.

Slavery on an industrial scale was a major source of the wealth of the British empire, being the exploitation upon which the West Indies sugar trade and cotton crop in North America was based. Those who made money from it were not only the slave-owners, but also the investors in those who transported Africans to enslavement. In the century to 1810, British ships carried about three million to a life of forced labour.

Campaigning against slavery began in the late 18th century as revulsion against the trade spread. This led, first, to the abolition of the trade in slaves, which came into law in 1808, and then, some 26 years later, to the Act of Parliament that would emancipate slaves. This legislation made provision for the staggering levels of compensation for slave-owners, but gave the former slaves not a penny in reparation.

More than that, it said that only children under six would be immediately free; the rest being regarded as "apprentices" who would, in exchange for free board and lodging, have to work for their "owners" 40 and a half hours for nothing until 1840. Several large disturbances meant that the deadline was brought forward and so, in 1838, 700,000 slaves in the West Indies, 40,000 in South Africa and 20,000 in Mauritius were finally liberated.