Traffic information service ROMANSE tweeted that the crash had happened on the westbound route just before the A3(M) junction.
Traffic queues are building, with drivers warned to expect delays of about 40 minutes from Emsworth.
It is not yet known how many vehicles have been involved in the crash, or whether there are any injuries.
Traffic is also slow on other major roads, including the M27, as drivers are warned to slow down and keep safe during the wet weather this morning.
More to follow.