The southbound carriageway is shut between junction 10a for Crawley, West Sussex, and junction 11 – the A23 link – for Pease Pottage and Horsham in the same county. Traffic is being taken off at junction 10 for north Crawley and sent onto local roads through the town, which are also congested as a result.
The accident involved a car and a HGV, which has also shed its load.
Queues are back to junction 9 for Gatwick Airport and Horley.
Those intending to travel to the airport – a hub for British Airways and EasyJet – should leave with extra time to complete their journey.
Northbound traffic is slow at junction 11 too, largely due to onlookers.
It is congested back along the A23 to Handcross, West Sussex.
Northbound drivers are not advised to leave the A23 as there is no diversion in place.
Sussex Roads Police tweeted: “#M23 southbound is completely blocked between J10a and J11 due to an overturned vehicle & debris.
“Traffic being diverted off: updates to follow when clear.”
A tweet from National Highways reads: “The #M23 is closed southbound between J10 near #TinsleyGreen and J11 #Crawley due to a collision involving a lorry and a car. The lorry has overturned and shed its load onto the carriageway.
“@sussex_police are in attendance. Thank you for your patience.”