A section of Southsea's promenade has collapsed as work gets under way to stabilise the sea defences.
The sea wall near the town's naval memorial was damaged by strong winds and high tides in November.
Its latest collapse is in an area already identified as dangerous by Portsmouth City Council where emergency repair work is taking place.
The council said 10m (32ft)-long sheet piles were being driven into the beach to protect the area from more damage.
The emergency works are due to be complete on Friday and further repairs will resume in the new year.
A permanent solution which involves rebuilding 4.5km (2.8 miles) of sea defences is expected to begin in 2020, pending approval by the Environment Agency which will unlock government funding.
Portsmouth's deputy leader Steve Pitt said: "We are aware that our current sea defences are no longer fit for purpose and close to the end of their maintainable life.
"Incidents like this underline the need to move forward with our new defence scheme as soon as possible."