The driver of a white van on the A34 has crashed into the central reservation.
The accident happened on the northbound carriageway shortly after 12.30pm, about a mile south of the Milton interchange.
Both lanes of the northbound carriageway are now closed while Highways Agency staff clear the scene.
It is not yet known whether the driver of the white Ford van, a man in his 40s, has been hurt.
But a spokesman for South Central Ambulance said the man had suffered a "medical episode" which caused him to crash.