Muslim pirates sailing out of Algiers raided all along the Mediterranean and out into the Atlantic as far as Madeira. By the 1620s, 10,000 European slaves were being held in the city's dungeons, including Scots, Irish, Dutch, Danish, Slav and Spanish captives. Others included Japanese and Chinese victims.

The Flemish aristocrat Emanuel d'Aranda, who spent two years as a prisoner doing punishingly heavy labour before he was ransomed, calculated that 600,000 European Christians were enslaved in Algiers between 1536 and 1640 alone. That tallies with the generally accepted estimate that a million white Europeans were enslaved from the 1500s to the 1800s.

Raids on coastal villages were horrific and bloody. Devon and Cornwall suffered repeated slave raids in the 1620s; and in 1627, two bands of slavers hit south-east Iceland, capturing more than 400 men, women and children. A man named Bjarni Valdason, who tried to escape, was clubbed over the head and killed, his body butchered into small pieces 'as if he were a sheep', according to one witness.

Houses were torched. One young mother and her two-year-old toddler were hurled into a blazing building and burned to death: 'When she and the poor child screamed and called to God for help, the wicked Turks bellowed with laughter. They struck both child and mother with the sharp points of their spears, forcing them into the fire, and even stabbed fiercely at the poor burning bodies.'

Those are the words of Olafur Egilsson, a Lutheran minister in his 60s, who was beaten until he could no longer stand, as the pirates tortured him to find out if the villagers had hidden treasure.

Distressing and deeply shocking as these individual stories are, they are a few cases among millions. The scale of slavery in the Muslim world was vast beyond imagination. 

'At one time,' the eminent historian Professor Robert Tombs says, 'everyone knew about it. It was one of the main hazards of Mediterranean commerce for Western sailors. But today, most people are completely unaware it ever happened.'

Partly, that is due to the current insistence that the British empire was the source of all historical evils. It does not suit the politically correct narrative to admit that Muslim slave traders were the scourge of Africa, long before the Europeans arrived . . . and long after they left. 

Citing the Encyclopedia Britannica, Marozzi estimates that in 1861, at the start of the American Civil War that would put an end to U.S. slavery, there were more slaves in the Muslim states of West Africa than in the Confederate states of the Deep South of America. 

The Arab slave trade dated back long before the beginnings of Islam in the 7th century. The Prophet Mohammed owned 70 slaves including Persians, Ethiopians, Copts (Christians from modern-day Egypt) and Syrians.

Between that time and the First World War, up to 17 million people were taken prisoner and used as slaves in Muslim armies and in brothels, on building sites and in private homes. That could be 50 per cent more than the total number of Africans transported across the Atlantic, a figure usually put at 11 million to 15 million.

By sickening tradition, the treatment of women was especially brutal. 

The castration of boys to make them into eunuchs was still practised as recently as the 19th century. The French aristocrat and explorer Count Raoul du Bisson saw it performed in Abyssinia (now Ethiopia), calling the operation 'barbarous and revolting'.

'The little, helpless and unfortunate prisoner, or slave, is stretched out on an operating table,' he wrote in 1863. 'His neck is made fast in a collar fastened to the table, and his legs spread apart, and the ankles made fast to iron rings; his arms are held by an assistant. The operator then seizes the little penis and scrotum, and with one sweep of a sharp razor removes all the appendages.'

A bamboo catheter was then inserted into the urethra, to prevent it from scarring over, and hot oil, honey, tar or mule dung smeared over the cuts. The boy, typically aged between six and 12, was buried in warm sand up to his neck to stop him from moving while his wounds healed.

A majbub, or eunuch without his penis, fetched a much higher price at slave markets than a khasi, one who had merely had his testicles removed. A khasi was more likely to serve as a soldier or policeman than a majbub, who could be trusted in the harem. British people 200 years ago were no less repulsed by such stories than we are today.

As well as leading the way in ending the transatlantic slave trade in the 19th century, Great Britain put intense pressure on the Ottoman empire in Turkey and the whole of the Arab world to end slavery.

'Even while suppressing the transatlantic slave trade,' says historian and ethicist Professor Nigel Biggar, 'the British empire was busy trying to suppress the Arab slave trade in Africa – especially East Africa – including using the Royal Navy to intercept slave ships between Zanzibar and the Middle East.'

But in much of West Africa, slavery continues today. 

l Captives And Companions: A History Of Slavery And The Slave Trade In The Islamic World, by Justin Marozzi, is published by Allen Lane.