Who is considered the biggest mole in British Intelligence?

so far I've noticed if I don't know the answer, the longest answer has always been right

The british lost all their credibility after he was exposed. But U.S. had the same problem when Aldridge Ames was caught spying. He caused the deaths of at least 30 U.S. double agents.

Wendell Thomas, Ivan Trump , American as borscht and blinis

Player Linny, yep me too for awhile now

how was he exposed