The motorway is closed both ways between junction 3 and 4 until Monday morning while the work is carried out.
For drivers travelling westbound Highways England has put a diversion in place over nearly 60 miles, taking vehicles up the M3, A34 and back down to Southampton via Salisbury.
There are delays on the A27 through Ashfield and North Baddesley this morning as drivers try to take a shorter route through nearby villages.
There is also traffic queuing on major routes in Romsey and Southampton.
Why is the work being carried out?
Highways England is repairing two bridges over the M27 carriageway.
The current bridges are said to beÂ '˜unable to move as originally designed' and are nearing the end of their lives '“Â with cracks appearing in the concrete beams.
The 40-year-old bridge is nearing the end of its life and is in need of replacement.
The Romsey Road Bridge is well used and carries tens of thousands of vehicles daily.
The bridge is actually made up of two individual bridges, both will be closed for three full weekends while the existing bridge is demolished and a new one installed.
The replacement bridge has a different design and will have a lifespan of 120 years. Â