A Toyota Prius collided with the back of a horse box near junction 5 (Guide Interchange) on the eastbound carriageway at around 4.50pm yesterday (Sunday, May 30).
Officers believe the Prius had travelled into the same lane as the horsebox to avoid a "small black vehicle", which had cut in front of it as the motorway merges from three lanes to two.
The passenger of the Prius, a 63-year-old woman from Blackburn was rushed to hospital with a "serious arm injury" where she remains.
The driver, a 69-year-old man from Blackburn, suffered minor injuries.
No horses were injured.
The black car did not stop and officers are urgently trying to trace it.
Investigating officer, Sgt Dave Hurst, said: "We need to find the vehicle involved, which we believe to be small and black, so we can speak to the driver.
"If you know who this is or you saw what happened, and you know the make and model of this car, please get in touch. Similarly, if you have any dash cam footage of this incident, we need to speak to you.
"Lastly, I am appealing to the driver of the car to do the right thing and come forward. It may be that you didn't realise what had happened and so please contact the police as soon as possible."
Anyone with information has been asked to contact police by calling 101, quoting incident reference number 1193 of May 30.
Alternatively, independent charity Crimestoppers can be contacted anonymously on 0800 555111 or online at crimestoppers-uk.org.
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