Flag of the Civil Defence Service, first flown in 1943. Some (possibly locally-manufactured) variants exist in 2:1 ratio and lacking the lettering and crown.
Source: Own work, incorporating some public-domain images, and based on an image at http://www.crwflags.com/fotw/flags/gb_cd.html
Transferred from en.wikipedia to Commons by Zscout370 using CommonsHelper.
Wally Wiglet at English Wikipedia