The motorway has been closed between Junction 8 and Junction 9.
Emergency services, including an air ambulance, are at the scene of the accident.
The northbound carriageway is blocked but a diversion is in place for southbound traffic.
Motorists have been warned of delays following the crash.
A Twitter user posted a dramatic picture of an air ambulance on the road.
They said: “M23 southbound closed before J9 due to accident. Air ambulance in attendance.”
Another person shared a picture of the traffic and wrote: “Huge accident on the M23 both sides closed been parked up for nearly an hour.
“Interesting the obvious delay of emerg services response due to this new no hard shoulder idea.
“At least 15 blue light vehicles had to weave through miles of traffic. Sad day for those involved.”
Highways England tweeted: “The #M23 southbound is now closed between J8 (#M25) and J9 (@Gatwick_Airport) due to a serious multi-vehicle collision.
“@SurreyRoadCops are on scene along with @SCAS999 @airambulancekss @SurreyFRS and our traffic officers.”
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