It is said that the English phrase "Keep calm and carry on." is related to a brief history of the British World War II. Please tell me its origin.

At the beginning of World War II in 1939, the British government designed three posters to inspire people. Keep calm and carry on was the slogan of one of the posters. However, these posters were never made public until 2000, when they were found in a second-hand bookstore. Discovered and posted

"Compared to 1939, when Nazi Germany swept across Europe, Britain was in a precarious situation in the face of Germany's military operations. Under the threat of a powerful enemy, staying calm is the most basic attitude. , persist in faith and keep moving forward, and it is work that cannot be stopped every day, so KEEP CALM AND CARRY ON has of course become the best slogan."——————He Feipeng's "keep calm and carry on"