Officers were called at 1.27pm to reports of a blue Peugeot 207 that left the northbound carriageway of the M3 near Winchester services and ended up at the bottom of a verge nearby.
Police closed one of two lanes while the emergency services dealt with the incident.
The driver, a woman in her 60s, sustained life threatening injuries and was taken to Southampton General Hospital by air ambulance. Her next of kin are aware.
The lane closure is still in place and causing long delays so police are urging drivers to find a different route.
If you saw the incident, or have dash cam footage showing what happened, please call police on 101 quoting 44210299307.
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