When I heard the government’s announcement that petrol and diesel cars are to be banned from 2040, I resorted, as I often do for entertainment, to the British Pathé news archive. I found a 1967 film showing trials of a prototype electric Mini, as well as a similar experiment from Ford. Then came this rather delicious prediction, delivered in clipped tones: ‘In the next few years there is the prospect of seeing millions of them on the road.’

The hype over electric cars has been going on a long time. Had Harold Wilson been moved by it and done what Michael Gove, the Environment Secretary, has just done, he would have passed a law banning petrol and diesel cars from 1990 — and the country would have been virtually immobilised when that year arrived. Why create a hostage to fortune now? The Conservative manifesto, published in May, suggested merely: ‘We want almost every car and van to be zero emission by 2050, and will invest £600 million by 2020 to help achieve it.’ Yet with the introduction of the government’s plan for tackling nitrogen oxide emissions, published last week, that position has rapidly evolved into a pre-announced outright ban, which echoed a declaration by the French government early last month.

I don’t have any repressed emotional need for the throaty roar of a V8 exhaust pipe. In common with millions of motorists I would happily swap my antisocial diesel Citroën estate for an electric model tomorrow were it not for two problems. First is the cost of batteries. Take the Nissan Leaf, the world’s best-selling electric car. Nissan’s website quotes a list price for the base model of £16,680. This, it turns out, excludes the batteries. You can either rent these from Nissan at a cost of £80 to £90 a month, depending on what mileage you cover, or you can buy the batteries at a further cost of £5,000. Nissan can only sell its electric cars at this price, however, thanks to a government subsidy of £4,500 per car. Without this subsidy, and including the cost of the batteries, your Nissan Leaf would cost £26,180.

Nissan doesn’t make a petrol version of the Leaf — perhaps to avoid a direct comparison — but the nearest equivalent, the Micra, retails at less than half this: £11,995 for the base, petrol model. No doubt there is scope to reduce the manufacturing costs of electric cars and batteries, but it is difficult to see them attracting willing consumers unless the industry can first overcome the other fundamental problem with electric cars: their range, or rather, lack of it.

According to the Nissan website, a Leaf will do 124 miles between recharges, after which it will have to be plugged in for a minimum of four hours (if you install a special recharging point in your garage, assuming you have a garage) or 12 hours from an ordinary electric socket. If you are able to find a rapid recharging point, a device which has been installed at a motorway service station, for example, it is possible to achieve an 80 per cent recharge in 30 minutes. It gets worse. The 124-mile range is only in test conditions. When the magazine What Car? tested the Leaf it found the car’s range in real road conditions to be between 70 and 80 miles. Top Gear tested a more upmarket model which claimed to have a range of 155 miles and found that it managed 90 miles. This was when the batteries were new. Over time, the charge they can hold tends to decline. Longer-term owners on various green websites have found their Leafs to cover only 45 miles (a vehicle which had covered 52,000 miles, tested at 70 mph) and 30 to 35 miles (in a car which had covered 90,000 miles).

Battery technology will improve, but you can’t assume that will happen fast enough to meet the 23-year deadline. After all, engineers have been grappling with the problem for 50 years. They managed to improve the range compared with the 1960s prototypes, which could only manage 35 miles between charges, but progress has stalled since 1996 when General Motors produced an electric car called the EV1, with a 100- to 140-mile range. The EV1 won devotees among celebrities and environmentalists — yet GM, which leased the cars rather than sold them, recalled the lot and crushed them.

The same devotees now flock around Tesla, the Silicon Valley company set up by the PayPal billionaire Elon Musk. The company has never made a profit, yet in April it overtook the $50 billion market valuation of General Motors. Tesla claims to have 400,000 pre-orders for what would be its first mass-market car, the Model 3 — its waiting list stretching into next year. Again, though, the problem is range. Tesla claims it will be able to travel 215 miles between recharges. As with Nissans, however, that is a little hopeful — one owner of a four-year-old earlier Tesla model, claimed to have a range of 245 miles, says he can’t get beyond 150 miles.

If the problems of range and battery cost can be solved, the government’s ban on petrol and diesel cars would not be a problem. But then neither would it be necessary, because motorists would go electric anyway. Mile for mile, running an electric car is already far cheaper than running a petrol car — Nissan suggests less than 2p a mile if the electricity is bought off-peak, compared with over 10p for petrol or diesel. Servicing costs are also markedly lower.

A lot of this disparity, however, is down to tax on road fuel — which accounts for 67 per cent of the price of a litre of unleaded.   If this revenue stream dries up, the government will have to find other taxes to impose on us. The low cost of electricity is thanks to the fact that most of it is still generated from cheap fossil fuels. From an environmental point of view, a switch to electric cars only makes sense if the electricity used to power them is produced by renewable means. National Grid, which has responsibility for the electricity distribution system, estimates that electric cars, should they become ubiquitous, will require a peak demand of between 6GW and 18GW — this on top of the 60GW peak demand which can currently be satisfied.

The lower figure assumes that owners of electric cars recharge their machines mostly with off-peak electricity. That seems unlikely, unless the range of the cars can be significantly extended. Extra power capacity can be built, but how much of it could be renewable? In 2016, only 9 per cent of electricity was generated by renewable means — and that includes waste burning, and the burning of wood pellets imported from the US, which cause about as much particulate pollution as diesel engines. Moreover, the more intermittent wind and solar generation that comes on stream, the more reliant we become on back-up sources of power — or vast banks of batteries. We now have 3.9GW back-up generating capacity provided by gas and, er, ‘diesel farms’ — ranks of diesel-powered generators. It would be somewhat ironic if the government managed to phase out diesel cars only to find that much of the electricity required to power them comes from diesel engines anyway.

Technology could change dramatically in 23 years. By then we might be able to drive 700 miles and then recharge in minutes. Or, like nuclear fusion, which has spent the past 50 years being just around the corner, electric vehicles may turn out to be the great hope which never quite materialises. We just don’t know. Given that, wouldn’t it have been a better idea to keep the abolition of petrol and diesel cars as an aspiration rather than to pre-announce a ban? The ban is an example of a novel form of policy-making which began with the Climate Change Act in 2008 — where government makes laws to take effect at some point in the future on the assumption that some uninvented technology becomes invented.

Nine years on from the Climate Change Act and with the 2050 target of reducing carbon emissions by 80 per cent now just 33 years away, we still have no idea whether it will be achievable. Defra wouldn’t quite answer my question asking when the 2040 ban was added to its plan for tackling pollution. Since it only appears in the introduction to the plan for reducing pollution, not in the body of the text, it has the air of a last-minute addition. Was it a pitch to appear virtuous on the international stage, or to emulate the French? Or was it a sop to car companies in order to secure post–Brexit investment? Remarkably, the announcement of a future ban on petrol and diesel cars came in the same week that, to the surprise of many, BMW said that the electric version of the Mini will be built in Britain.

Remember how Nissan announced last October that it would commit to building new cars in Britain, without anyone being sure what the government had promised in discussions with the company? That’s the same Nissan which staked $5.6 billion on developing the Leaf — many of which are built in Sunderland. How convenient that instead of Nissan having to compete for custom, motorists are suddenly going to be forced to buy the product in which the company has a market lead.

We will just have to hope that the electric car hype finally comes good. If not, don’t bother planning a long road trip after 2040.

Ross Clark and Isabel Hardman discuss electric cars on the Spectator Podcast.