Joseph P Walshe, then secretary-general of the Department of External Affairs, proposed the move in a handwritten letter to De Valera from Khartoum.

"Will you give a little thought to the question of a colony when you have leisure, " he asked, adding that "it would be a splendid training ground for our people and colonial budgets can be made to balance without subsidies from the home government."

Walshe's letter is included in the latest volume of papers on Irish foreign policy which will be published this week by the Royal Irish Academy (RIA) in Dublin.

The civil servant began his letter to De Valera with the admission, "I came down here by flying boat just a week ago, and have done quite a lot of exploring in spite of a shade temperature of 105 degrees during the day and 95 degrees at night." Such frankness between a government official and a politician would be highly unlikely in the modern era.

Walshe also tells of meeting the Governor General of Sudan:

"an idealist as is his wife . . . who is a R Catholic. . ." He observed that "the people who get blacker and blacker, as you go further south are invariably very courteous and you are not in the least disturbed at not meeting any white people."

The RIA book also includes a confidential report by an Irish official in London following a meeting with Britain's King George VI at which he jokingly said he did not like being called an "organ" in Dail debates on the 1937 constitution. The British monarch also said his late father had praised De Valera's "rare gift of natural good manners" but regarded Winston Churchill as "like the cat that walks alone; he must go off and do things on his own without any real attempt at cooperation with his fellows."