The A34 has been closed in both directions near Sutton Scotney following a road traffic collision.
Police and other emergency services are currently at the scene of the incident, which has resulted in the closure of both the northbound and southbound carriageways.
The closure is in place between Junction 9 of the M3 and Bullington Cross Roundabout, where the A34 meets the A303.
Police are urging motorists to avoid the area and find alternative routes while emergency crews deal with the incident.
The closure is likely to affect traffic travelling between Winchester, Andover and the A303, with motorists advised to allow additional time for their journeys.
Hampshire and Isle of Wight Constabulary said emergency service partners were working at the scene and thanked motorists for their patience and understanding.
Police have said a further update will be issued once the road has fully reopened.
The circumstances surrounding the collision, including whether anyone has been injured, have not yet been confirmed.
Air Ambulance involved at least 5 Ambulances 3 + Fire engines, paramedic cars & too many police & highway vehicles to count involved