How did Pius XII warn the Allies about the planned German invasion of the Low Countries?

In May 1940, Pope Pius XII warned the Allies of the impending German invasion of the Low Countries.1

While Pius XII maintained a policy of impartiality throughout the war—insisting that his position was not one of neutrality, which he equated with indifference—this stance did not prevent him from providing intelligence to the British government regarding German military plans. In addition to the warning about the invasion of the Low Countries, he also informed the British early in 1940 that certain German generals were prepared to overthrow the Nazi government if they could be guaranteed an honorable peace.1