The first hull loss was the BOAC Comet 1 G-ALYZ on take off from Rome Ciampino on 26 Oct 1952, when the aircraft aborted takeoff after failing to gain height. The captain in command was blamed for the accident and demoted from flying Comets, but he was later exonerated from blame as it was the characteristics of the aircraft that caused the accident.
The first fatal crash was the Canadian Pacific Comet 1 CF-CUN
at Karachi on 3 March 1952, whilst on a flight proofing run. Apart from being the first Comet crash with fatalities, this accident also led to cancelled orders from the likes of Canadian Pacific, Pan Am etc.
The two crashes mentioned in Reply 2 were the most devastating that put the whole programme back. However, they were not the first accidents due to structural failure. This was BOAC Comet 1 G-ALYV near Calcutta on 2 May 1953, barely two months after the Canadian Pacific Comet crash at Karachi. The exact cause of the crash of G-ALYV was never conclusively established. Some of the wreckage was removed by locals before the investigators arrived, and the initial investigation was botched by the Indian authorities.
Despite the false start of Comet service, the Comet 4 was a damn fine aircraft but by the time De Havilland had ironed out the bugs, Boeing had taken the lead with their 707s. There were further crashes involving Comet 4s, none of which were due to the design of the aircraft. In fact many Comet 4s were subjected to heavy batterings and bad flying but it was the strength of the design that prevented more accidents happening.
I flew on the Comet 4Bs of BEA and Olympic Airways, at least 4 or 5 times. Partly because of my young age at the time I hated the aircraft but looking back, she was sure unappreciated and was really a diamond in her elements.
MOL on SRB's latest attack at BA: "It's like a little Chihuahua barking at a dying Labrador. Nobody cares."