Transport for London has been forced to keep the A4 Hammersmith flyover closed for further engineering inspections because of serious corrosion of steel supporting cables.
External concrete spalling has been known about for several years, but maintenance contractor Amey has called for extra time to assess the extent of damage within the core of the 900m bridge structure.
It now looks likely that the busy flyover will need some heavy-duty repairs to extend its life.
The flyover was closed on 23 December due to concerns about a serious structural defect.
Since engineers and contractors have been working round the clock on a detailed investigation.
They hope to decide shortly on what remedial action needs to be taken and when the flyover can safely be reopened to traffic.
The damage to the ageing 1960?s structure has been caused by salt water from grit laid during winter de-icing. This has corroded and weakened the internal supporting cables.
Alarm balls rang when Amey engineers decided to break out two sections of concrete around pre-stressed tendons close to a pier head and revealed they had snapped.
Engineers now need to test the strength of each tendon to get a picture of the current strength of the bridge.
TfL had planned to renovate the bridge after the Olympics, but now it may be forced to take emergency remedial action on what is a key route from Heathrow to the Olympic stadium
Engineers are exploring all options to reopen the flyover to traffic as soon as possible, but must await the outcome of further work to test the extent of the problems found in the structure.
Leon Daniels, TfL’s managing director of Surface Transport, said: “Our team continues to work night and day alongside the world’s leading structural engineers to fully understand the extent of the flyover’s structural problems.
“I have been inside the flyover and seen for myself the unique issues we face,” he explained.
“Safety must be our top priority and we have not taken the decision to close the flyover lightly.
“However, we are working flat-out to determine what measures we must put in place to safely reopen the flyover as soon as possible.”