The incident, involving a 62-year-old man, happened in Broadford Bridge Road, West Chiltington, at about 8.50am.
The man, from Littlehampton, suffered serious head injuries and was taken to the Royal Sussex County Hospital in Brighton for further assessment and treatment, where he remains today (Monday).
A spokesperson for South East Coast Ambulance Service said: “Two cyclists reportedly collided.”
However, Sussex Police said the man fell from his bicycle.
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