Such a move, anathema to purists, would plainly have saved BL money. But Jaguar's top engineers showed outstanding insubordination in designing the XJ6 with an engine bay too narrow to accommodate a V-format. That scuppered BL's plans. But it also meant that to fit a V12 would require massive front-end surgery.
Now, six-and-a-half years after the XJ6 made its debut to a rapturous press, it has sired the V12. With twice as many cylinders, and though not exactly twice as desirable as the XJ6, the new car is the most outstanding Jaguar saloon of recent years.
It immediately overcomes two Jaguar weaknesses, compared with its German and Japanese competition: poor performance and poor engine refinement. The XJ12 is not only sports-car fast; it also has a turbine-like smoothness that makes high-speed motoring marvellously unfussed.
In essence, it is the XJ6 fitted with a fettled version of Jaguar's long-running aluminium V12, first introduced in the E-type of 1971; this V12 is larger (6.0 litres instead of 5.3), 22 per cent more powerful (pumping out 318bhp), and its new four-speed automatic gearbox provides smoother gear changing, and thus even more seamless progress.
You may well identify an XJ12 by its speed, but not by its looks. It is virtually indistinguishable from an XJ6. The grille is black, not silver, the badging - including discreet V12s on the boot and glovebox - is gold coloured; but that's about it. That a pounds 46,600, 6.0-litre car should look the same as its pounds 26,200 3.2-litre sister makes it either a fabulously discreet performance car, or a luxury vehicle lacking in snob appeal, depending on your view.
On the road, the XJ12 is staggeringly fast. It erupts from traffic lights and, unfettered by speed limits, would cruise at 120mph-plus all day, with a top speed of 155mph. The only other big car I have driven that is so brisk is the V12 Mercedes 600SEL. More important, though, the XJ12 purrs calmly around town.
Jaguar saloons - all Jaguar saloons - are marvellously soothing. You can feel your worries disappear when you waft along in a Jag. Only a Rolls-Royce does it better.
Whereas big Mercedes or BMWs are hard and efficient, Jaguars are soft and cushioning. They are lounge rooms, not offices; luxury, not merely executive, cars - a difference that the Germans and Japanese have never properly grasped.
Moreover, there is an enormous price gap between the Jaguar and its very costly German rivals. This gap will widen further next year, when the Budget decision to tax company cars according to price, rather than engine size, comes into effect.
Jaguar cabins are beautifully fitted out with wood and leather. No matter how the Germans and Japanese try, they just don't make wood look good in cars; one theory is that British cars tend to have flatter interior surfaces which lend themselves better to the use of wood.
Mind you, the XJ12 is not quite as balm-like as previous V12-powered Jaguars. They traded performance for quietness, handling sharpness for ride suppleness, and everyday practicalities for sheer styling beauty. They were absolutely single-minded in their quest for luxury.
This XJ12 is harsher. Its engine is audible, whereas on some old V12 Jaguars you could swear that the motor had stalled, so quietly was it idling. And you can hear some tyre slap, and feel some bumps, even at low speeds. Older Jaguar saloons moved over uneven roads like big liners gently cruising over the ocean.
This, then, is a cat of a different character. It's sportier, sharper, leaner, fitter. Ford, now holding the purse strings, is backing an attempt to dump Jaguar's 'old man' image, and to improve the company's share of the rich German market, where high performance is coveted and Jaguars sell poorly.
That the XJ12 still manages to soothe so sweetly as well as stimulate so strongly is an achievement of which once-embattled Jaguar should be proud.
Jaguar XJ12 pounds 46,600. V12 engine, 318bhp, four-speed automatic gearbox. Top speed 155mph, 0-60mph in 6.8 seconds. Average fuel consumption 15.8mpg.
BMW 750iL pounds 62,250. Looks rather dowdy now, and its V12 engine isn't as refined as the Jaguar's. Still, it boasts strong performance and lots of room. Watch out for horrendous depreciation.
Daimler Double Six pounds 51,700. The XJ12's twin. Same engine, but uses softer suspension, and less sporty styling. Rides softer, handles sloppier.
Lexus LS400 pounds 39,999. Quiet, fast and refined, but lacks the presence of its European rivals. Disappointingly plasticky cabin is at odds with its Mercedes-beating aspirations.
Mercedes-Benz 600SEL pounds 88,600. In all-round competence, probably the world's best car. Let down by its sheer (and unnecessary) bulk, dull styling and slightly bland character.