A woman suffered leg and chest injuries when her car crashed into a country park visitor centre.
The impact almost demolished a corner of the listed building at Exceat, near Seaford, and left the roof and walls in danger of collapsing in the road.
Firefighters had to free the woman, from Crowborough, who was trapped in her car following the accident at Seven Sisters County Park visitor centre yesterday.
The woman was taken to Eastbourne District General Hospital, but police said her injuries were not thought to be life-threatening.
PC Greg Allcorn said: "We are worried about the building collapsing. It's only a couple of feet away from the road and it is dangerous as it stands at the moment because the wall could easily fall into the path of traffic and take the roof with it."
Police called council workers to bring in a set of temporary traffic lights while the damage was repaired.
It could cause months of traffic jams on the busy A259 between Eastbourne and Seaford.
Paul Walton, area manager for the South Downs Conservation Board which manages the centre, said: "It would appear it has caused serious structural damage."
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