How would early Yamatos change Allied battleship design?

If four Yamato-class battleships were completed early, how quickly could Britain and the United States respond?

It depends on when they learned the ships' real characteristics; early warning permits crash construction, while later discovery shapes the next generation instead.

Discovery timing is the decisive variable. If Britain and the United States learned before the war that four Yamato-class ships were ready, they could begin an emergency counterbuilding program. Wartime discovery would leave less time for a direct answer, and British intelligence initially believed the ships carried 17-inch rather than 18-inch guns. Studying the class after capture or defeat could still push Britain toward enlarged Lion-class successors with 18-inch weapons. The response would emerge through intelligence and industrial lead time rather than matching a specification overnight.