Hampshire Police's Sergeant Rob Heard said: “This is a very serious incident that has sadly resulted in the deaths of two people and my thoughts are with both families.
“Our investigation is focusing on establishing exactly what happened and the circumstances that led to the collision.
“We know the Clio was travelling west but for reasons yet to be established it crossed into the path of the Megane which was travelling in the opposite direction.
The Clio was struck by the Megane on its nearside causing fatal injuries to the front-seat passenger, Kiri-Jade Hodgkinson.
“I would like to hear from anyone who witnessed the collision, who saw either of the vehicles driving prior to it or anyone else with information that could assist our enquiries.”
Anyone with information is asked to contact Sgt Heard at Eastleigh police station on 101, quoting Operation Mohite.