The insurance giant Geico must give more than $5m to a woman who had sex with a motorist in his car and contracted a sexually transmitted disease, a Missouri appellate court ruled.

The ruling represents a preliminary legal victory for the plaintiff over the company best known for commercials starring an anthropomorphized gecko which speaks with a British accent.

Geico can still go to the state supreme court to seek a reprieve, and may get a more favorable ruling in a related federal case.

The plaintiff – identified in court records as “MO” – alleged that in 2017, during sexual encounters in a 2014 Hyundai Genesis, her boyfriend infected her with a virus that causes genital warts.

The woman accused the man of acting negligently and argued that the Geico policy which insured the car should cover her “injuries and losses” from the disease.

Geico denied coverage of the claim and rejected a settlement offer from the woman. Court filings say the woman and her boyfriend agreed to arbitrate her claims, the official overseeing that process determining the man “negligently infected MO” and awarding her $5.2m in damages.

The woman went to a state court in Jackson county, Missouri, and filed suit against Geico, aimed at confirming the arbitration award. That court ruled in her favor, prompting Geico to ask an appellate court to overturn the decision.

Attorneys for Geico argued that they never had a meaningful chance to contest the claim. The appellate court found Geico did get the chance, an opportunity it forfeited when it chose to simply deny coverage.

“We would note that [Geico] had every opportunity to enter a defense … but chose not to do so,” said the ruling, issued by judges Edward Ardini, Karen King Mitchell and Thomas Chapman. “Geico [has] no right to relitigate those issues.”

Geico and MO have a separate but related case pending in federal court. In that proceeding, the insurer contends that the policy in question “only applies to bodily injuries arising out of the ownership, maintenance or use” of the Hyundai Genesis.

“MO’s alleged damages have no nexus to the ownership, maintenance or covered use of the … Genesis,” Geico’s lawyers have argued in federal court filings. “Her injuries arose from … her failure to prevent transmission of [sexually transmitted diseases]” by engaging in sex with someone carrying the virus causing genital warts.