The work, between Junction 3, Nursling, and Junction 4, Chilworth, will add an extra lane in each direction to improve traffic flows and cut congestion on this stretch of the M27, which carries around 121,000 vehicles daily.
The new lanes will be added entirely within the existing boundary of the motorway and the widening work has been carefully planned to keep as many lanes as possible open for drivers.
Minister for Roads Tom Harris, welcomed the start of works warning there would be "some short term disruption" adding that the Highways Agency would be working hard "to keep this to a minimum".
Work on widening the 4.1km stretch is expected to take 15 months, with the road opening to traffic in Spring 2009. During the work, three narrow lanes will be kept available in each direction during peak periods. A 50mph speed restriction will be in place for the safety of roadworkers and the travelling public and will be enforced by average speed cameras.