The A31 is currently closed in both directions between Bentley and Alton after a 'serious road traffic collision' involving two cars.
Hampshire Constabulary says officers were called at 12.27pm after the crash on the westbound carriageway and have warned the road "will remain closed for some time".
The closure has also caused widespread gridlock in the lanes either side of the A31, through Bentley village and Froyle, and in Binsted and Holybourne.
A police spokesman added: "Thank you to all motorists in the area for their patience."