Sussex Police say the M23 is likely to remain closed southbound for some time after a stationary car was hit by two other vehicles early this morning.
Four people have been taken to the Royal Sussex County Hospital suffering from serious injuries.
The hard shoulder has reopened but the rest of the carriageway is likely to remain closed for some time while investigations continue.
People travelling to Gatwick Airport are being advised to follow the diversion routes.
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Wednesday's weather for the east of the region
Wednesday's weather for the west of the region