The motorway has been shut since around 1am following the collision, which is thought to involve two cars.
Highways officials say it is likely to remain closed during the morning rush, while investigation work is carried out.
A diversion is place, with drivers being told to follow the solid square symbol.
This will go via the A251, A20 and A249 through Challock, past Leeds Castle and through Maidstone.
The road was reopened shortly before noon.
Avoid getting stuck on Kent's roads by keeping up-to-date on all of the latest traffic and travel news here or by following our live blog.