Presented by Rene Cutforth

On 17 January 1917 the code experts of British naval intelligence intercepted a telegram from the German Foreign Office that could decide the outcome of the First World War.

This amazing section of Admiralty codebreakers - known as Room 40 - had already helped to win a major naval battle and catch both Mata Hari and Sir Roger Casement. Now the Zimmermann Telegram was to test Room 40 to its limits.

Taking part are the former head of Room 40, Admiral Sir William James, and one of the first lady codebreakers, Miss May Jenkin - better known as Aunt Elizabeth of Children's Hour.