Highways England are beginning work to install a new concrete barrier and draining improvements on the motorway.
More than 2,000 metres of a concrete barrier will be placed between junctions 6 and 7.
New drainage will be carried out between junctions 3 and 7.
The work will take about six months to complete.
Speed restrictions and reduced lane widths will be put in place.
The clockwise carriageway between junctions 5 and 7 will be reduced to 40mph on the M621.
The anti-clockwise carriageway will be reduced to 50mph from M1 junction 43.
Most of the work will be carried out overnight between 8pm and 6am.
All lanes will be kept open during the day.
-> Inquest into death of Libby Squire set to open todayM621 junction 3 to M1 junction 43 will be closed overnight in both directions for the duration of the work.
There will be diversion routes in place.
This is the second part of a project – which began in 2017 – which is improving safety for drivers travelling along the M621.
The project has so far seen 3,401 metres of concrete barrier installed in the central reservation, drainage work carried out between junctions 7 and 3 and a steel safety barrier introduced at the side of the motorway.
Highways England project manager Sujad Hussain said:
“These are vital safety improvements on a busy commuter route. We thank drivers for their patience and encourage them to allow extra time for their journeys, especially during rush hours.
“We will not only renew the safety barriers in the central reservation and verges, but also fully resurface the carriageway in both directions, further increasing safety.”