Did the situation described in Imitation Game ever happen?

Question

In the movie Imitation Game, there is a scene where a message is decrypted. In this message, a German submarine says he's going to attack a ship: the decoders decide not to warn the ship because they are afraid the Germans could understand that Enigma is cracked, and they don't do anything.

From the Wikipedia page on the movie, it is said that the decision was not taken by the decoders of Bletchey Park themselves but at higher levels.

My question is: Assuming that it was taken at the "correct" high level, was such a decision ever taken?

My research about this on internet showed no results saying it was taken, for example here.

Also, I know about several devices and tactics of WW2 that makes unlikely such a decision:

• German submarines would follow procedures and not communicate to high command during an attack, especially when underwater. Thus, the German high command would not know about how the submarine was lost, so concluding that Enigma was broken would have been far-fetched
• British navy used in WWs "Q-ships", which where used as decoys apparently easy to attack but suddendly revealing themselves as submarine chasers
• Zigzags were followed by convoys to avoid being intercepted easily, thus changing route to avoid being attacked by a submarine would not raise alert to the Germans
• Even if Bletchey knew a submarine was about to attack, air or naval reconnaissance was still needed to locate precisely and destroy the submarine. Thus, to use Enigma intelligence needed to use other sources of intelligence that could justify the destruction of the submarine
• Also, not sure that they knew it by the time, but Allied merchant ships production was more important than losses and the rate of progression of German submarine production was not so good, thus performing attrition right now on German submarines was better than waiting for an hypothetical "big battle" where plenty of German submarines would be destroyed thanks to Enigma (wolfpack tactics grouped only ten or twenty German submarines)

Answer 1

Yes it happened

There is a paper from "The Naval Historical Society of Australia" with title "Ultra and the Battle of the Atlantic - The True Story" in which are reported book sources (I found it really interesting).

In early 1943, Admiral Ernest J. King, U.S. Chief of Naval Operations (CNO), pushed to use U-boat location information derived from Ultra to attack Germany’s tanker U-boats (known as Milch Cows).

… in 1943 the changing locations of these Milchcows became available to us through Ultra. By this time too, there was very close liaison between Coastal Command and their American counterpart who were especially able to cover the southern Atlantic convoy routes. With typical enthusiasm the Americans were all for sinking the Milchcows at once; they were, however, persuaded by Jack Slessor, Commander-in-Chief Coastal Command, not to give Doenitz cause to suspect Ultra and the Milchcows were quietly sent to the bottom over a reasonable time; it must have been one of the greatest blows suffered by the U boats.

book source: Winterbotham, F.W., The Ultra Secret, Futura Publications Limited, A Contact Book, page 111.

It is interesting how to disrupt the u-boat menace, where attacked the tanker, i.e. attack on logistic instead of u-boat themselves

Answer 2

No, such decisions were never taken.

You already answered your question partially:

• Submarines wouldn't communicate when attacking
• Q-ships would contact through their chain of command

Bletchey Park would need time, and a lot of it, to decode a message. At least a couple of hours, more likely a lot longer, like a full day.

They would send the transcripts to the relevant higher authorities, not the people in the field. The higher authorities would decide what to do, and pass it down through the chain of command. This was a fairly automatic process, but it would take time.

By automatic I mean their were procedures: In the Mediterranean the Royal Navy was under strict orders not to attack based on Enigma information alone (for those who knew about it), unless confirmed by other means. Cunningham (those under him probably didn't know about Enigma) was clever enough to send an reconnaissance patrol immediately in the general area, to make the Germans believe a plane discovered the ships, rather than code breaking.

That makes it very unlikely that decrypted Enigma documents were used tactically. The time required both ways made it only useful strategically. (with thanks to @T.E.D.)