12.25pm update: all westbound lanes have reopened.
The shoulder remains closed, and motorists are advised to prepare for delays of up to 45 minutes.
Original story below.
Two westbound lanes of the M4 Motorway are closed after a three-car collision.
Emergency services are at the scene, and motorists have been advised to expect heavy traffic.
Traffic was backed up by six kilometres at noon, as traffic slowed past the crash site.
All east-bound lanes remain open.
It is not yet known whether anyone was injured in the collision.