What is it?
The Jaguar C-X75 supercar. Which won’t be finished. And won’t be sold. And yet in 2013, the year of the supercar, it was all set to turn the triumvirate confluence of LaFerrari, McLaren P1 and Porsche 918 Spyder into the most awesome foursome that the car world has ever known.

The Jaguar now looks destined to become the forgotten giant, after a decision taken by the company in December 2012 not to put the extended-range petrol-electric two-seater into production after all.

The irony is that it might have signalled much more than a million-pound Ferrari or McLaren: newfound ambition for a once world-beating British marque again willing to compete right at the top of the food chain. A marque once again looking to take a guiding hand in the development of the state of the automotive art. Something of a renaissance, in other words.

More’s the pity. As things stand, Gaydon’s supercar experiment is over. Five working prototypes exist, and there are no plans to make more. Whispers persist that a few of them may be auctioned, but nothing’s confirmed. Strange circumstances for a first drive – but, in this case, we’ll take ‘em.

In supercar terms, the C-X75 moved from apparently fanciful show car to fully operational validation prototype very quickly – and changed quite a lot on route.

Those who last read about this car after its unveiling as a concept at the Paris motor show of 2010 will be wondering where its tiny jet turbine power generators have gone. Somewhere along the line, Jaguar concluded – just as Ferrari, McLaren and Porsche did – that the supercar isn’t quite ready to part with reciprocating pistons just yet.

What was decided, in May 2011, was that the buzz surrounding the C-X75 concept car was too great to ignore. The car would go forwards, engineered in partnership with Williams Advanced Engineering.

But, like the show car, it couldn’t be just another supercar. It had to be as fast as a Bugatti Veyron. It had to emit less carbon than a Toyota Prius - sub-90g/km, as things stood back then. It needed a zero-emissions range as good as a Chevrolet Volt. And it needed to look like the original show car.

It wouldn’t be enough for this car to breach the bounds of possibility in just one direction – the familiar direction: speed. The C-X75 had to push the envelope in opposing directions simultaneously, on performance and fuel efficiency.

What is it like?
In place of the Bladon Jets omnivore turbines came a primary powerplant that would set Jaguar’s engineers a similar challenge on cooling, and allow it similar freedoms on packaging. Developed in-house by Jaguar, the C-X75’s 1.6-litre petrol four-pot is all-aluminium, and is like no small-capacity engine ever intended for the road.

Fitted with both a supercharger and a turbocharger, it produces unbelievable power for its size: an astounding peak 502bhp at 10,000rpm. And because the C-X75 is a plug-in hybrid, that engine’s only half the story.

Immediately behind the driver – who’s positioned almost perfectly between the front and rear axles – there’s a 19kWh lithium ion battery pack capable of supplying a continuous 300kW of power.

The car’s electric motors are Jaguar’s own. They’re the size of cake tins, there’s one for each axle, and they produce 194bhp and 295lb ft each. They also only weigh 20kg, making them more efficient, judged on output per kg, than any electric motor Jaguar could buy in.

The one up front drives the wheels directly through reduction gearing; the one at the rear runs in parallel with the engine, sending power through a seven-speed automated manual gearbox to the rear wheels.

And so, running at full chat, the C-X75 produces in excess of 850bhp, and has 738lb ft of torque. It’ll accelerate to 60mph in less than 3.0sec, to 100mph in less than 6.0sec, and go on way beyond 200mph.

Scarcely believably, it also produces less than 89g/km on an NEDC emissions test, and drives for 40 miles on battery power alone. And it looks incredible – more like the rightful heir to Malcolm Sayer’s C- and D-types, and the elegant XJ13, than either the XJ220 or the XJR-15 ever seemed.

You could fill textbooks explaining the innovative engineering in this car. The all-carbonfibre construction makes for torsional rigidity of 60,000Nm per degree – three times greater than a Lamborghini Murciélago.

Every major mechanical and electrical component is positioned within the wheelbase, with the exception of the seven-speed gearbox – which goes in sideways to minimise the overhang behind the rear axle.

The thermal management systems are ridiculously complicated, as they’d have to be in order to make happy bedfellows of a large battery (which operates best at 31 degrees) and a 502bhp, 10,000rpm engine (which exhausts at up to 900 degrees). Both, by the way, are surrounded by a carbonfibre engine bay that, in places, would begin to unbake itself at 200 degrees or so.

In the pouring rain at its Gaydon UK headquarters, Jaguar gave us limited opportunity to get familiar with its technical prodigy. Some passenger laps on the twisty inner handling circuit suggested the C-X75 has supremely manageable limit handling for a supercar. “We went to a lot of trouble to give the car Jaguar feel,” says driver and Williams chassis chief Simon Newton. And you know what he means.

The car does skids. “The normal power split in EV mode is 70 per cent biased for the rear wheels, and we limit power at the front wheels when cornering because it tends to bring on understeer. We’ve also worked out a few tricks with the E-Diff to add some throttle-steer, and – when it’s on – the ESP functions similarly to McLaren’s ‘brakesteer’ to keep the nose tucked in on corner entry."

In electric mode, the performance level feels strong – if limited. Instant, torque-dominated: a bit like a turbo hot hatch but entirely without the lag. I can’t tell you what the electric motors sound like, because they’re drowned out by the C-X75’s sound synthesiser, which fills the cabin with an electronic noise somewhere between a whistle and a loud whine. It’s not unpleasant, and maybe it does make the electric mode feel more dramatic. You’d never mistake it for ‘real’ noise, though.

My turn at the wheel. Engaging full-fat hybrid mode and moving off, that inline four suddenly announces itself. It’s all chattering gear-driven cams and bad-tempered low-rpm grumble to begin with, but the accelerator pedal’s tamely progressive thanks to that supercharger.

Might as well flatten it then. We’re in third gear, on the high-speed circuit of Jaguar’s Gaydon HQ, where mile-long straights allow some close inspection of the C-X75’s outright speed – specifically, of the potency of that powertrain. At 3500rpm the barp of exhaust begins to emerge over all that chatter.

At 6500rpm, the engine finally seems fully awake and starts to really howl. There’s no lump of mid-range torque, no breathless top-end – laudable flexibility, in fact. And there’s an incredible red zone where, at 8000rpm, the engine hits a show-stopping full stride. At which point you’ll forget all about the electric motors, carbonfibre and engineering genius, and find yourself totally caught up in a sense of pure mechanical interaction. Perhaps this Jaguar is an old-school supercar after all.

After several full-power blasts, a picture emerges. Even in the rain, the C-X75 feels every bit as fast as they say it is – up to a point. Up to about 120mph, to be precise - to the top of fourth gear, until which point it could probably run with a Veyron. At least very close to one.

But beyond 150mph, the C-X75 doesn’t surge onwards with quite the same urgency. It’s effortlessly fast but, in the highest range, doesn’t keep going like the very fastest in the world. It doesn’t need to be travelling well into three figures before it really opens up, like a Veyron.

All I can put it down to is that the electric motors don’t seem to give their best at big speeds. And that 503bhp isn’t quite enough – however spectacularly it’s made – to make up the shortfall.

Should I buy one?
Well, you can't. But driving the car leaves you with the impression that the C-X75 project has probably ended up exactly where it should be, because would supercar owners understand that, to appreciate their new million-pound car, they have to stand back and see the bigger picture?

Would they be able to understand that it may not quite be the ultimate machine in the most vivid sense, but that there’s more to it than sheer speed? How many Veyron owners know how much CO2 their car emits? Don’t they just want the fastest car in the world?

Maybe. In order to create the supercar that does it all, perhaps Jaguar had to take the customer out of the equation. The company might have been braver. But equally, maybe it’s not such a bad thing that it wasn't.

Because, while it may not quite be the fastest car in the world, the C-X75 is still a modern, daring kind of machine. A hypercar, really – if such a term were ever truly justified by a supercar that does more.

It acknowledges that, in the 21st century, there is no part of the car market untouched by the need for environmental responsibility – nor can there be. And, like the Porsche 918 Spyder, it proves there’s a genuine zero-emissions solution than can still produce absolutely first order speed and excitement.

Price N/A; 0-62mph less than 3.0sec; Top speed 220mph; Economy tbc; CO2 less than 89g/km; Kerbweight 1700kg; Engine 4 cyls, 1600cc, twincharged petrol, plus 2 x 194bhp electric motors; Installation mid, longitudinal, four-wheel drive; Power 850bhp+ at 10,000rpm; Torque 738lb ft Power to weight 500bhp/tonne Specific output 313bhp/litre (IC engine); Gearbox 7-spd robotised manual