A chunk of land the size of a swimming pool has collapsed into the River Wear - amid fears it will happen again.
A 25m stretch of the Panns Bank quayside fell into the water overnight on Tuesday.
Now officials admit another 25m stretch of the quayside is "destabilised, severely weakened and very likely to fail."
However Sunderland City Council insist there is "no danger" to the public, with the area already fenced off.
Council contractors started working on the area back in January after cracks appeared in the pavement and roads alongside a section of the quayside.
Inspections were carried out to help draw up a condition survey that would then inform a future works programme and plans for the rebuilding of the quayside wall.
However work suddenly stopped on Tuesday when staff reported movement in the backfill and in the quayside piling.
"Overnight on Tuesday 16 March a 25 metre section of the quayside and its piling collapsed into the river," confirmed a council spokesperson.
"A further 25 metre section of the quayside is also destabilised, severely weakened and very likely to fail.
"While investigations on causes are continuing, initial conclusions are that water ingress on both sides of the piling weakened it and contributed to the collapse.
"The works area was already fenced off and there is no danger to the public.
"Subject to further ground investigations and surveys, the council anticipates that a 60 to 70 metre section of the quayside will need to be rebuilt.
