Construction company Knights Brown has a £22.3m contract to carry out the work and is expected to finish by the end of 2026.
Knights Brown previously constructed the first and third phases, meaning they will have completed the majority of work on this scheme.
In total, the whole scheme covers 8.4km of coastline between Tipner and Milton. Most of the area is low-lying and many of the current coastal defences were approaching the end of their effective lives.
The fifth phase includes 1.3km of flood defences along Ports Creek, the northern shore of Portsea Island.
The design consists of a sloped earth embankment, which provides the required flood defence level, with a rock armour toe to dissipate wave energy, therefore preventing further breakup of the shoreline.
The project is funded by the Environment Agency and implemented by Coastal Partners on behalf of Portsmouth City Council.
Portsmouth City Council leader Steve Pitt said: "It's an exciting time as we forge towards completing the final link in this vital £85.9m sea defence project, which is made possible thanks to funding by the Environment Agency."
The scheme as a whole is one of the largest local authority-led coastal flood defence projects in the UK and is crucial for the protection of Portsmouth residents. The defences are designed to safeguard more than 4,000 homes and almost 500 businesses.
Got a story? Email news@theconstructionindex.co.uk
