Fifty years ago, in the experimental department of the Jaguar factory in Coventry, a new engine burst into life. It was the XK engine and it was destined to power some of the most evocative cars of this century. It was an engine which would speed Jaguar cars to five historic victories in the Le Mans twenty four hour race. It was an engine which would power generations of luxury saloons. It remained in production for over forty years and this year Jaguar will celebrate the fiftieth anniversary of the birth of an engine which helped transform the company from an ambitious but specialist car builder into one of the world's premier luxury manufacturers.

The first glimpse the motoring public had of the twin overhead camshaft XK engine was at the London Motor Show, at Earls Court, in October 1 948. It was installed in the car which was the undoubted star of the show - the Jaguar XK 120.

The XK story, though, had started some years earlier when Jaguar's founder, William Lyons, had been on firewatch duty during the war. Together with his Chief Engineer, William Heynes, and Claude Baily, he decided that Jaguar needed a powerful, refined new engine for the generation of post war Jaguar cars he was planning. The XK engine would be the first-ever engine designed by Jaguar's own engineers. William Lyons had previously used adapted versions of other manufacturers' engines in his pre-war SS cars.

Heynes, Baily, Walter Hassan and cylinder head specialist, Harry Weslake, worked on the new engine in the immediate post war years at Jaguar's factory in Swallow Road, Coventry. Although originally designed in both four and six cylinder versions, only the 3.4 litre, six cylinder engine would eventually go into production. The two litre, four cylinder version though was listed in the original XK catalogue and a prototype four cylinder XK engine powered Major A.T.G. 'Goldie' Gardner's EX 135 streamliner to 175 mph on the Jabbeke Highway in Belgium in August 1948.

Hailed at the 1948 London Motor Show" the most beautiful production sports car in the world", the XK120 quickly confirmed William Lyons' proud contention that it was also the fastest production car in the world. The Jaguar boss conclusively demonstrated this in 1949 when he took a standard XK120 and a planeload of British motoring correspondents back to Belgium's Jabbeke Highway. Driven by Jaguar's test driver, Ron 'Soapy' Sutton, the XK120 covered the flying mile at 132.596 mph.

Performance credentials established, the Jaguar XK120 and its purpose-built racing cousins, the XK-powered C-type and the D-Type, went on to score famous victories in races and rallies in the UK, Europe and North America. Most memorable of all were the great wins in the Le Mans twenty four hour endurance race in the 1950s. Jaguar C-Types were victorious in 1951 and 1953 and then the D-Types gave Jaguar three more wins in 1955, 1956 and 1957.

In addition to dominating the race tracks in 1950s, the XK engine proved its versatility by powering Jaguar's luxury saloons. The compact Mark 1 and Mark II saloons relied on XK power as did the elegant, imposing Mark VII, Mark VIII and Mark IX saloons. And the sporting successors to the original XK120 - the XK140, the rare and uncompromising D Type-based XKSS and the XK150 - had XK engines under their bonnets.

Thirteen years after the original XK engine made its debut came a car which was destined to become one of the greatest sports cars of all time. The Jaguar E-Type was launched at the Geneva Motor Show in 1961 - immediately becoming an icon in its era. Under the long, sensuous bonnet was the XK engine, now enlarged to 3.8 litres and giving enough power to rocket this sensational car to 150 mph.

Whilst sports car enthusiasts savoured the performance of the XK engine in the E-Type, businessmen around the world relaxed as the XK engine's refined power quietly propelled their luxurious Jaguar Mark X limousines.

In 1964, the XK engine was again enlarged - to 4.2 litres - giving added power to the sporting E-Type and even greater refinement for Jaguar's luxury saloons. Smaller capacity versions of the engine though continued to be available for the 240 and 340 saloons and the much admired S-Type saloon.

In 1968, exactly 20 years after the debut of the XK engine, came a new Jaguar saloon which set new standards of style and refinement. The Jaguar XJ6 was immediately hailed as one of the world's best luxury saloons with its combination of sports car performance and handling, and limousine luxury.

From its launch in 1968 until the last XK powered XJ6 was built in 1986, this car remained a benchmark for modern luxury saloon car design. Contemporaries of Sir William Lyons say that the XJ6 was the car of which he was most proud and the last photograph taken of Jaguar's founder shows him standing at the gates of his Warwickshire home alongside a Jaguar XJ6.

The last XK-engined Jaguar saloon - the Series 111 XJ6 - went out of production in 1986. But XK engined cars were still available into 1990s. The stately Daimler limousine, which was first introduced in 1968 had always relied on XK power and this imposing vehicle continued to be built, in strictly limited quantities, until 1992.

The engine that was first conceived by Williams Lyons and his talented team of engineers in the dark nights of the Coventry blitz, and was first unveiled to the public in 1948, was installed in Jaguar and Daimler production cars for 44 years. It started its distinguished service under the bonnet louvres of a high performance sports car. It ended its historic 44 year production run in a limousine which is today used regularly by Her Majesty Queen Elizabeth II.

Jaguar's Chairman and Chief Executive Nick Scheele said: "The XK engine was a masterpiece of design and the car in which it was first installed, the Jaguar XK120, quite clearly signposted the direction Jaguar would follow in the future. Few engines have made such a profound mark on the history of the motor industry and it is a fitting legacy of Sir William Lyons and his enormously talented team of engineers. It is wholly appropriate that we celebrate the fiftieth anniversary of the XK in a year when we announce the latest member of the Jaguar XK family, the Jaguar XKR powered by a supercharged version of Jaguar's new AJ V8 engine. This car and this engine are in the finest traditions of Jaguar and its proud engineering heritage."