From the War Cabinet minutes, 3 September 1939, meeting at 5pm:
Consideration was given to the question whether we drop propaganda leaflets over Germany. It was believed these leaflets would have an important effect on German public opinion. Moreover, the Germans would realise that British aircraft were flying over their country. The dropping of leaflets might well have its maximum effect in the first few hours of war. The risk to pilots was not regarded as great. The area in which it was proposed to cover were Hamburg, Bremin and the Ruhr.
The War Cabinet agreed that its operations to be carried out during the forthcoming night.
The War Cabinet agreed to —
(a) to authorise the immediate dispatch of a bomber force to attack the German fleet reported to have sailed from Wilhelmshaven that afternoon;
(b) to authorise the dropping by aircraft of propaganda leaflets on Germany on the night of the 3rd – 4th of September;
(c) that —
(i) the Chief of the Air Staff should inform General Gamelin of the above decision was taken by the War Cabinet, and
(ii) that the French Ambassador should be similarly informed and requested to reaffirm to the French government the intention of His Majesty’s Government not to initiate air action which might involve the risk of civilian casualties.