A Florida mother and her young daughter found dead in a Porsche on a Florida highway may have been poisoned by a faulty car battery.

The bodies of Latifa Lincoln (46) and three-year-old daughter Maksmilla Lincoln were found on the Florida Turnpike in Osceola County near Orlando on 2 June (2016).

Her car bumped into a guardrail and came to a full stop, and when emergency services arrived at the scene they found the engine running and the radio on.

Inside the 2006 Porsche Cayenne SUV, they found the two bodies and a smell so strong three rescuers were hospitalised with breathing difficulties. The smell was said to be a "foul caustic chemical odour." It was soon apparent the pair had not succumbed from crash injuries.

One theory was that the victims had succumbed to carbon monoxide, but now investigators have found they had breathed in large quantities of another gas, hydrogen sulfide, a colourless but highly flammable gas which occurs naturally and smells of rotten eggs.

It is suspected the gas may have leaked from the car's battery, which was not the original for that model.

Unlike in most cars, where the battery is stored under the bonnet, in a Porsche Cayenne the battery is stored under the driver's seat. It is unclear if this contributed to the deaths or why the battery had been replaced.

Lincoln had purchased the vehicle a month before from a nearby dealership called Auto Express in East Orlando, the Mail Online reported. Finance manager Ron Telleysh told local news channel WESH 2: "We feel sorry for the lady. She used to come in here all the time with her little daughter."

Assistant Orange-Osceola Medical Examiner Dr. Gary Utz told WESH: "It's unprecedented. I haven't been able to find another case." Utz has sent the battery away to the National Highway Traffic Safety Administration for further tests before officially blaming the car battery for the two tragic deaths.