2 May 1952
The British Overseas Airways Corporation (BOAC) was the British state airline from 1939 until 1946, and the long-haul British state airline from 1946 (later merged with British European Airways Corporation to form British Airways). In 1952, BOAC became the first airline to introduce a passenger jet, the de Havilland Comet. On 2 May 1952 the first regular jet flight, with a Havilland Comet, left London with 36 passengers on board. It arrived in Johannesburg 18 hours later. The London-Johannesburg route therefore became the first regular service flown by a jet airliner.  
References

Wallis, F. (1952), 'Worlds First Turbojet Passenger Service' from Geneology, [online], Available at: freepages.genealogy.rootsweb.ancestry.com [Accessed 21 April 2009]