“Delay is the deadliest form of denial” was published by C. Northcote Parkinson (1909-1993) in The Times (London) in 1966, with the title of “Parkinson’s Law of Delay.” An administrator can deny something by delaying action on it, and thus not having an official record of saying “no” to the proposal. The law also appeared in Northcote’s book, The Law of Delay: Interviews and Outerviews (1970), but did not originate there.
 
     
Wikipedia: C. Northcote Parkinson
Cyril Northcote Parkinson (30 July 1909 – 9 March 1993) was a British naval historian and author of some sixty books, the most famous of which was his bestseller Parkinson’s Law, which led him to be also considered as an important scholar within the field of public administration and management.
 
15 September 1966, The Times (London), “Parkinson’s Law of Delay” by C. Northcote Parkinson:
There is nothing static in our changing world and recent research has tended to show that the Abominable No-man is being replaced by the Prohibitive Procrastinator. Instead of saying “No” the Prohibitive Procrastinator says “In due course,” foreshadowing Negation by Delay. The theory of Negation by Delay depends upon establishing a rough idea of what amount of delay will equal negation. If we suppose that a drowning man calls for help, evoking the reply “In due course,” a judicious pause of five minutes may constitute for all practical purposes, a negative response. Why? Because the delay is greater than the non swimmer’s expectation of life. The same principle holds good in a case at law. Delays are thus deliberately designed as a form of denial and are extended to cover the life expectation of the person whose proposal is being pigeon-holed. Where the urgent matter requires remedial legislation, delay takes on a new dimension. The judicious pause will correspond, nevertheless, to the life expectation of the man from whom the proposal originates. DELAY IS THE DEADLIEST FORM OF DENIAL.’
 
1 October 1966, The Salt Lake Tribune (Salt Lake City, UT), pg. 20, col. 5:
Delay Is Deadly Denial
“Delay is the deadliest form of denial,” writes Cyril Northcote Parkinson, expounder of Parkinson’s laws, in a supplement of The Times of London. The Prohibitive Procrastinator (PP) doesn’t say “no” to a new idea. He forms a committee to study it. And eventually he wins without taking a real negative stand.
 
11 October 1966, Greensboro (NC) Record, “Parkinson’s THird Law” (editorial), pg. A-14, col. 1:
Now, Professor Parkinson is still at work in the forests of bureaucracy. His latest law has just been issued. It is: “Delay is the deadliest form of denial.”
 
8 September 1967, Omaha (NE) World-Herald, “Bon Mots” by Robert McMorris, pg. 14, col. 1:
This is one of the latest philosophical one-liners conceived by Larry Shomaker, senior vice-president of Northern Natural Gas Company, who dreams up such witticisms in his spare time.
 
Some other additions to the list of “Shomaker’s Laws.”
 
. Don’t drive a tack with a sledge hammer.
. Delay is the deadliest form of denial.
 
OCLC WorldCat Record
The law of delay : interviews and outerviews
Author: C Northcote Parkinson
Publisher: London : John Murray, 1970
Edition/Format:   Book : English
 
23 May 1971, Seattle (WA) Times, “Delay and conquer” by John Haigh, pg. E5, cols. 7-8:
“THE LAW OF DELAY” by C. Northcote Parkinson. Houghton Mifflin Co. $4.95.
(...)
Parkinson’s new law can be stated: “Delay is the deadliest form of denial.”