> In his 1974 book The Ultra Secret, Group Captain F. W. Winterbotham asserted that the British government had advance warning of the attack from Ultra: intercepted German radio messages encrypted with the Enigma cipher machine and decoded by British cryptoanalysts at Bletchley Park. He further claimed that Winston Churchill ordered that no defensive measures should be taken to protect Coventry, lest the Germans suspect that their cipher had been broken.[1] Winterbotham was a key figure for Ultra; he supervised the "Special Liaison Officers" who delivered Ultra material to field commanders.[2]
> However, Winterbotham's claim has been rejected by other Ultra participants and by historians. They state that while Churchill was indeed aware that a major bombing raid would take place, no one knew what the target would be.[3][4]
> Peter Calvocoressi was head of the Air Section at Bletchley Park, which translated and analysed all deciphered Luftwaffe messages. He wrote "Ultra never mentioned Coventry... Churchill, so far from pondering whether to save Coventry or safeguard Ultra, was under the impression that the raid was to be on London."[5]
> Scientist R. V. Jones, who led the British side in the Battle of the Beams, wrote that "Enigma signals to the X-beam stations were not broken in time," and that he was unaware that Coventry was the intended target. Furthermore, a technical mistake caused jamming countermeasures to be ineffective. Jones also noted that Churchill returned to London that afternoon, which indicated that Churchill believed that London was the likely target for the raid.[6]
4. Hunt, David (28 August 1976), "The raid on Coventry", The Times: 11
5. Calvocoressi, Peter (1981). Top Secret Ultra. New York: Ballantine Books. pp. 85–86. ISBN 0-345-30069-6.
6. Jones, R. V. (1978). Most Secret War: British Scientific Intelligence 1939–1945. London: Hamilton. p. 149. ISBN 0-241-89746-7. This book was also published in the US under the title The Wizard War.
All that being as it may (because you're right), don't let that detract from the larger point about the tension between using intelligence products (like a tip-off about a bombing raid) and protecting the sources and methods of those intelligence products for the future.
The British were paranoid about this; if they wanted to attack a tactical target where the Germans would think "there's no way they could have known about this, our codes must be broken", they'd always send a decoy scout or something out first to "accidentally come across" the secret target.
The Americans did similar things; when Adm. Yamamoto was shot down, his flight was duly "detected" by a reconnaissance plane first, even though the Americans knew full well when it was taking off and where it was going.
> However, Winterbotham's claim has been rejected by other Ultra participants and by historians. They state that while Churchill was indeed aware that a major bombing raid would take place, no one knew what the target would be.[3][4]
> Peter Calvocoressi was head of the Air Section at Bletchley Park, which translated and analysed all deciphered Luftwaffe messages. He wrote "Ultra never mentioned Coventry... Churchill, so far from pondering whether to save Coventry or safeguard Ultra, was under the impression that the raid was to be on London."[5]
> Scientist R. V. Jones, who led the British side in the Battle of the Beams, wrote that "Enigma signals to the X-beam stations were not broken in time," and that he was unaware that Coventry was the intended target. Furthermore, a technical mistake caused jamming countermeasures to be ineffective. Jones also noted that Churchill returned to London that afternoon, which indicated that Churchill believed that London was the likely target for the raid.[6]
Source: https://en.wikipedia.org/wiki/Coventry_Blitz#Coventry_and_Ul...
References:
1. Winterbotham, F. W. The Ultra Secret, London, Weidenfeld & Nicolson, 1974 ISBN 0-297-76832-8; also London, Futura, 1975, ISBN 0-86007-268-1
2. Ray, John, "The Night Blitz", Cassel & Co 1996, ISBN 0-304-35676-X p. 155
3. "Defending Coventry" (http://www.historiccoventry.co.uk/blitz/defend-cov.php). Historic Coventry.
4. Hunt, David (28 August 1976), "The raid on Coventry", The Times: 11
5. Calvocoressi, Peter (1981). Top Secret Ultra. New York: Ballantine Books. pp. 85–86. ISBN 0-345-30069-6.
6. Jones, R. V. (1978). Most Secret War: British Scientific Intelligence 1939–1945. London: Hamilton. p. 149. ISBN 0-241-89746-7. This book was also published in the US under the title The Wizard War.