The work to junctions three and five represents a £12.5m investment and will reduce congestion, improve journey times, improve flow across the junction and increase safety.
The upgrades are part of the Government's £317m pinch point programme on motorways and major trunk roads to be delivered by 2015.
Specific work undertaken at junction three includes widening the M27 westbound exit slip-road to three lanes and widening the M271 northbound and southbound approaches to the interchange to three lanes.
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Further works involve the removal of the eastern overbridge hard shoulder to allow the new lanes and full signalisation of the interchange.
Highways England Asset Manager, Philip Sheppard, said: "It's fitting that on the 40th anniversary of the opening of the first stretches of the M27, we are able to increase capacity on these two busy junctions. This will allow the roundabouts to work more efficiently to reduce congestion and improve journey times. The widened approaches and new traffic signals will improve the flow of traffic for the thousands of drivers at both junctions."
The works, part of the biggest programme of road enhancements since the 1970's, will be completed in time for the August Bank Holiday.
(LM)
