Sussex Police are appealing for witnesses following the incident on Bellfarm Lane in Uckfield on Monday 16 January.
Police were called to reports of a woman injured in the road at around 7.30am. The 18 year old cyclist had significant injuries and her bike had damage consistent with a road traffic collision.
It's believed that the cyclist had been involved in a collision with a grey Audi A3 which failed to stop, she was air lifted to hospital with serious injuries.
A 35 year old man from Uckfield has since been arrested on suspicion of failing to stop after a road accident and causing serious injury by dangerous driving and is currently on bail while enquiries continue.
Police would like to speak to anyone who has information which could help with the investigation, particularly anyone who has any CCTV or dash-cam footage between 7.20am and 7.45am, and especially from any HGVs in the area at that time.
Contact firstname.lastname@example.org, quoting Operation Cradle with any information.