A cyclist was riding along Tanhouse Road, at the junction with Elmstead, when he came off his Dawes mountain bike at around 1.30pm yesterday (Monday, August 31).
Emergency services rushed to the incident where the cyclist, a 42-year-old man from Burscough, was found with potentially "life-threatening" head injuries.
He was taken to hospital by ambulance before the road was closed for three hours while specialist collision investigators attended the scene.
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Police say they do not believe any other vehicles were involved, but they are urging anybody who witnessed the incident or the cyclist in the moments before to get in touch.
Sgt Tracey Ward, of the Lancashire Police TacOps Team, said: "The cyclist has come off his bike while riding along Tanhouse Road and in the process has suffered extremely serious injuries. He remains in hospital and our thoughts are very much with him.
"Although we do not believe any other vehicles were involved, we are still trying to piece together exactly what led to the incident.
"Maybe you saw what happened, saw the cyclist in the moments before or perhaps you have dashcam footage taken in that area at around the same time.
"Whatever information you have, we would urge you to get in touch straight away."
Anybody with information has been asked to get in contact with police by calling 101, quoting log number 754 of August 31.