Hampshire Police say that the A34 is likely to remain closed in both directions for most of the day as repairs are made to the road surface following a serious accident overnight.
It's understood that a transporter lorry hit the central reservation near Whitchurch in Hampshire, causing the cars it was carrying to fall onto both the north and south bound carriageways.
Police and Highways Agency staff have been at the scene since 11:10pm last night.
No one was hurt in the accident and no other vehicles are believed to have been involved.
Drivers hoping to use the A34 today are being advised to take alternative routes.