Young women face higher car insurance rates if the European Union (EU) outlaws gender-based pricing of premiums.
If EU judges rule that charging men higher premiums breaches EU rules on sex discrimination, female drivers aged between 17 and 25 could see insurance costs soar by about 25%, according to the Association of British Insurers.
The ruling would also see rates for men fall by about 10%.
A campaign group, Open Europe, said the hike would equate to an extra payment of £4,300 on average for women and a saving of £3,250 for men over the same period, with insurance costs skyrocketing to as high as £9,300 for women in the worst cases.
As female drivers are considered a safer bet than males owing to their better road safety records and higher life expectancies, it is standard practice across Europe to base car insurance premiums on gender differences.
It is specifically permitted in EU anti-discrimination rules, which allow member states to discriminate on insurance rates and benefits "if sex is a determining risk factor, and that can be substantiated by relevant and accurate actuarial and statistical data."
But an Advocate-General at the European Court of Justice has advised judges that the concession in the EU "Gender Directive" is countermanded by "higher-ranking" equality provisions set out in the Charter of Fundamental Rights in the Lisbon Treaty.
If that legal "opinion" is upheld in Tuesday's final verdict, it will mean insurers can no longer gender-based different prices on a range of products including car insurance, private medical insurance, pension schemes and annuities.
Open Europe research director Stephen Booth said: "Giving EU judges free reign to rewrite laws that the UK Government has signed up to in good faith can cause hugely damaging and unforeseen consequences. That these judges would magically rule that young women should pay more in the name of equality is simply perverse. Instead of making prices fairer between men and women, this ruling would increase costs for consumers taken as a whole."
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