How worried was Britain about invasion 1940-41?

British government plans relating to a possible invasion, March 1941

Catalogue ref: INF 1/887

Dear Norman Brook,

With reference to your letter of the 14th February to Colonel Hollis on the plans for getting the public keyed up to a proper state of preparedness, I have held up the answer to your letter until the Chiefs of Staff have considered a report by the Vice Chiefs on the scale and likelihood of invasion.

2. The Chiefs of Staff consider that the Germans could not mount a full scale invasion without giving a number of indications of their final preparations. Starting from the position at the present moment, they think we shall obtain these indications during a period of about two or three weeks prior to the earliest date at which the expeditions could be launched. It must be emphasised, however, that the fact that all preparations have been made does not necessarily signify that invasion will be launched immediately, or even at all. It might be kept mounted for a considerable period, during which it could start at any time and at a few hours notice. We cannot be sure of getting information of the start of the expeditions.

3. The chiefs of Staff are anxious that a voluntary flow of evacuation from the coastal areas should begin from about the 1st of April, as weather conditions after this are favourable for invasion. They recommend that the public should have been made “invasion conscious” by this date and that they should also be made to realise that the danger period for invasion may well last throughout the summer months.

4. Invasion, when and if it comes, may be launched suddenly, once the final preparations have been made. The public must therefore clearly understand that no last minute evacuation will be possible. The motto from the 1st April should be “Go now or stay put”.

Yours sincerely,

Norman Brook, Esq.,
Ministry of Home Security
Whitehall, S.W.1