Sir Winston Churchill's family urged him not to convert to Islam after he became fascinated with eastern culture as a young man.

The revelation is contained in a letter from Lady Gwendoline Bertie, his future sister-in-law, written in August 1907, in which she urges him to rein in his enthusiasm.

In the letter, discovered by Warren Dockter, a history research fellow at Cambridge University, she pleads: "Please don't become converted to Islam; I have noticed in your disposition a tendency to orientalise [fascination with the Orient and Islam], Pasha-like tendencies, I really have."

In the letter, Lady Gwendoline, who married Churchill's brother Jack, continues: "If you come into contact with Islam your conversion might be effected with greater ease than you might have supposed, call of the blood, don't you know what I mean, do fight against it."

Churchill fought for the British Army in Sudan, and on India's north-west frontier, where he was exposed to the religion.

His fascination with Muslim culture led him to befriend William S Blunt, a poet who espoused Muslim causes. The two reportedly wore Arab robes in private in each other's company.

Dockter Warren Docker, a history research fellow at Cambridge University, found the letter while researching his forthcoming book, Winston Churchill and the Islamic World: Orientalism, Empire and Diplomacy in the Middle East.

Churchill held the then progressive view that Christianity and Islam should be viewed as equal. Dockter told the newspaper that his enthusiasm led to clashes with Imperialists.

"[…] During this period Churchill was in the Liberal phase of his career, having switched to the Liberals in 1904," Dockter told the Sunday Telegraph.

"He often came to loggerheads on imperial policies with hard-line imperialists such as Frederick Lugard, the High Commissioner of Northern Nigeria. Churchill was opposed to Lugard's punitive expeditions against Islamic tribes in northern Nigeria."

In 1941 when serving as prime minister he backed a £100,000 grant to be used to build London's first mosque in Regent's Park in the hope of securing the support of Muslim nations during World War Two.

However, Dockter said that Churchill's views of the religion were mainly influenced by Victorian romanticised notions of Bedouin culture and admiration for the Ottoman Empire, and there was little real likelihood of him converting.

His attitude to Islam was nothing if not mixed, and in his 1899 account of his experience fighting in Sudan contains highly critical accounts of Islam.

"The fact that in Mohammedan law every woman must belong to some man as his absolute property, either as a child, a wife, or a concubine, must delay the final extinction of slavery until the faith of Islam has ceased to be a great power among men," he wrote.

"Individual Moslems may show splendid qualities, but the influence of the religion paralises the social development of those who follow it. No stronger retrograde force exists in the world. Far from being moribund, Mohammedanism is a militant and proselytizing faith."