The driver of the Citroen, a woman in her 80s from Croston, suffered serious injuries to her pelvis and ribs as well as internal injuries.
She was taken to Royal Preston Hospital for treatment, police said.
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The driver of the Vauxhall, a woman in her 30s from Tarleton, suffered bruising.
Detectives said they were keen to trace a local post office worker who was believed to have witnessed the collision but left before police arrived.
Sgt Steve Hardman, of Lancashire Police's serious collision investigation unit, said: "I would encourage anyone who saw what happened to please make contact with us as soon as possible.
"We believe a post office worker witnessed the collision - if that was you, please get in touch."
Anyone with information can contact police by calling 101 or emailing [email protected], quoting log number 0577 of January 7.
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