Testosterone-fuelled young males are, all the statistics suggest, a danger on the road. Thrill-seeking young men are prone to drive too fast, late at night, and cause horrific fatalities. Young males are 10 times more likely to be killed or injured than a driver aged over 35. The typical insurance claim by a young male adds up to around £4,500, compared with £1,200 when the driver is middle-aged, whether male or female.

It's the main reason men pay around 40% more than women for car insurance until the age of 40, when accident rates and claims tend to equalise between the sexes. Yet from 21 December, insurers will be banned from taking gender into consideration when setting premiums – and women will pick up the tab.

According to the AA, the typical insurance premium for a male aged 17-22 is now above £3,000, while for a female it is £2,125. Men aged 23-29 pay an average of £1,840, compared with £1,200 for women. Only once men reach the age of 40 do their premiums come down to the same level as women (at around £700-£750).

What will happen when the directive comes into force? According to Gocompare.com, Britain's women are in for a shock. Most are unaware of the ruling and the huge impact it is likely to have. Gocompare's head of motor insurance, Scott Kelly, says: "We expect to see premiums equalised at the higher male rate rather than the lower rate for females. If that is the case, women drivers will see their premiums rise by just over £300 on average, but for younger age groups the increase could be as much as £2,000."

The scale of the premium increase – on top of the inflation-busting rises of recent years – means some young women who do not have to renew their insurance until January may save money by terminating their policy before the year end and starting a new one before 21 December.

But for older women, as Michael Winner might say, it's a case of "calm down, dear..." Unisex insurance rates will have virtually no impact on men and women above 40-45 as they are already treated as near identical risks at that age. Indeed, there may even be a very small bonus: the AA's figures suggest that women over 50 pay about £10 a year more in car insurance than men, so they may be in line for a small reduction. Young men will also benefit, with estimates that they will see 10% or more clipped off their premiums.

For now, the major insurers are silent about how they will price their policies for the unisex era. Money asked Direct Line and Aviva for quotes, but they refused to give specific figures, citing competition reasons.

Aviva would only go so far as saying: "Young males are likely to see their premiums fall, and young female drivers are likely to see their motor premiums rise." At Direct Line, Gus Park, director of motor insurance, said: "We were disappointed with the ruling, but now it is going to become law we're focused on minimising the impact of the gender directive. It's hard to be sure what is going to happen to prices within the market. Many customers will not be adversely affected. However, people shouldn't panic; they should wait for their renewal quote from their insurer and contact them if they have any concerns."

Many people will be justified in asking why the insurers don't just split the difference, with male and female insurance rates equalising halfway between the current rates. But Kelly explains: "If a 20-year-old male is being correctly priced for the risk he presents, then the insurer won't want to charge less than that. Instead, women will have to be charged more."

However that would suggest a huge surge in revenue for car insurance companies from their female customers. In reality, the Association of British Insurers doesn't believe all premiums will rise to the higher male rate, though it does think they will head that way. Overall, it reckons premiums for males will fall by up to 10%, but rise for females by around 25%.

A research paper for the ABI last year argued that the ban on the use of gender will result in an overall loss for consumers, as insurers will have less risk-based information to go on and will thus charge more overall. It also said the ban will not mean all women and men pay the same. "For example, a 27-year-old male driver from Swindon who drives a 2-litre BMW, and travels 15,000 miles each year, would need to be offered the same premium as a 27-year-old female driver from Swindon who drives a 2-litre BMW and travels 15,000 miles each year. This does not mean that, on average, male drivers in the insurer's book would be charged the same premiums as females."

But one thing insurers can't do is ignore the ruling. The ABI fought a long battle to maintain insurers' exemption from EU gender equality rules, but were finally defeated in a European Court of Justice ruling in March last year. The court gave insurers until 21 December to bring in unisex rates, with the biggest impact expected in Britain, Spain and Ireland, where gender-based pricing is most commonly used.

Ian Crowder, a spokesman for AA insurance says: "The ruling abandoned fairness in favour of equality. We were one of the voices saying it shouldn't happen, but it has, the ECJ has ruled, and that's now the end of the debate."

Supporters of the ruling argue it was never fair that gender should have come into premium calculations – plenty of young males are safe, cautious drivers, and plenty of young females are tearaways. Why should either be judged on gender? Instead, what the ruling is likely to spark is a switch to "telematics", where your premium is based on the data transmitted from a mini-computer in your car, which will record when you drive, where, and at what speed, as well as how hard you brake, or even corner. That may mean young males will still be charged a lot more than young females, but it will be because of driving profile, not gender.

Telematics, also known as "pay as you drive", is still in its infancy in the UK, making up only 3% of motor policies sold so far, and an early experiment by Aviva was less than successful. But as young females in particular face much steeper premiums, more will be tempted to opt for insurance based on telematics. A telematics deal usually starts off as relatively expensive, but as data builds up about the driver's habits and style, safe driving is rewarded by lower costs, with potential premium refunds. As Coverbox, a telematics insurer, says: "The box also allows us to check your miles, the times driven, where you keep the car overnight, and if it is different then we can decrease or increase the premium accordingly during the year."

Aviva is trialling a telematics "app" called RateMyDrive on smartphones, which can monitor acceleration, braking and cornering as well as the number of miles driven and the car's location. Safer drivers will be offered discounts of up to 20% off their insurance, although currently the trial is limited to 5,000 motorists.

Meanwhile, the EU gender directive affects all insurance products, not just car insurance. So from 21 December it will be illegal to use gender as a basis for discriminating in the pricing of pension annuities and life insurance. Women will again lose on life insurance; historically they pay lower premiums as they enjoy longer life expectancy. But there is a small payback when they hit pensionable age. Currently women receive lower annuities than men because they live longer, but the ruling will mean women may have to be given higher pension incomes.