A 37-year-old Brighton woman was airlifted to Kings College Hospital, London, after she came off her bicycle on the A275 near Lewes Racecourse on Friday May 3.
Sussex Police and emergency services were called to Waldergrave Road at around 11:50am.
Inspector Steve Grace from East Sussex Road Policing Unit said: “The cyclist remains in hospital in a serious but stable condition. “She is being treated for head and facial injuries. I would like to thank the passers by who came to the aid of the injured woman and called for an ambulance.
“We do not believe no other vehicle was involved but would like to hear from anyone who may have seen the cyclist prior to her coming off her bike. Please call 101 quoting serial 577 of 3/5.”