The authority said a riverbed survey found the cause of the problem was below the water level, where it appeared the river had been carving out a new, deeper channel on the riverbed.
Works were required as the water movement risked “undermining” the foundations of both the old bridge and the temporary footbridge, the council said, which posed “a real risk” to the stability of the whole structure.
It added the “complexity” of water movements needed to be assessed by specialists before any solution could be considered and it would be working with the Environment Agency over the coming months to agree the best way forward.
“Unfortunately, this means that the bridge will be closed over the holiday period and beyond,” the spokesperson said.
“This remains a live situation where decisions have been taken with the information we have.
“We fully appreciate the continuing impact this will have on residents, and especially businesses.
However, following analysis of the data from the investigations on Tuesday, we know that public safety must come first and this is the right decision.
“The businesses at Darley Abbey Mills and Darley Abbey village remain open as usual, and can be accessed via Haslams Lane. We encourage customers to continue to support them.”