Four people were seriously injured in a motorway crash after a stationary car was hit by two other vehicles.
Emergency crews were called to the southbound stretch of the M23 at junction 9 near Gatwick at about 4.12am today following the crash, Sussex Police said.
West Sussex Fire and Rescue Service had to cut one man free from one of the vehicles and all four casualties were taken to the Royal Sussex County Hospital in Brighton, East Sussex.
A police spokesman said: "Four occupants of the vehicles were seriously injured and one of them is in a critical condition.
"No further details of them or their conditions, or the circumstances, are available at present."
The southbound carriageway was immediately closed and is expected to remain shut until later today while forensic examinations take place, a police spokesman said.
He added: "The scene is still being examined and arrangements have been made for recovery of the vehicles.
"The hard shoulder was opened shortly after 10am. Police and the Highways Agency are grateful for the patience of drivers at this time. The carriageway will be fully re-opened as soon as possible."
He said the Highways Agency had advised drivers to follow the diversion route which is in place and that passengers for Gatwick Airport should remain with their vehicles and follow the diversion route.
The investigation is at an early stage but anyone who saw what happened is asked to contact Sussex Police on 101 quoting serial 0280 of August 26.