Since the revelation of the Ultra secret in 1974, it has been widely accepted that Ultra intelligence—that is, high-grade Axis codes decrypted by a cen- tralized British interservice unit called the Government Code and Cypher School (GC and CS) at Bletchley Park—gave Great Britain a decisive advantage over its Axis foes and that this advantage was particularly significant in the battle against shipping to North Africa.
O’Hara, Vincent P. and Cernuschi, Enrico
"The Other Ultra,"
Naval War College Review: Vol. 66
, Article 9.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol66/iss3/9