A letter from Sir Winston Churchill's future sister-in-law has been discovered which shows his family had concerns that he might convert to Islam.

The letter from Lady Gwendoline Bertie, written in August 1907, was found by Cambridge academic Dr Warren Dockter, the Telegraph reports.

Lady Gwendoline, who married Churchill's brother Jack in 1908, writes: "Please don't become converted to Islam; I have noticed in your disposition a tendency to orientalise, Pasha-like tendencies, I really have."

She adds: "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."

Dockter told the Telegraph that Churchill had fought in Sudan and India and so had experience of being in "Islamic areas".

The wartime Prime Minister had great respect for Islam, as well as admiration for the military prowess of the Ottoman Empire.

He even dressed in Arab clothes occasionally in private with his friend, the poet Wilfrid S. Blunt, who was a great admirer of Arab culture.

But despite his enthusiasm for the culture, his family's concerns were not borne out in reality.

In an interview with The Independent, Dockter said: "Churchill never seriously considered converting. He was more or less an atheist by this time anyway. He did however have a fascination with Islamic culture which was common among Victorians."

Dockter came across the letter while researching his forthcoming book 'Winston Churchill and the Islamic World: Orientalism, Empire and Diplomacy in the Middle East'.