The Highways Agency has said the M4 will remain closed in both directions between junctions 2 and 3 after a crack was discovered at the Boston Manor Viaduct.
It discovered the damage to the bridge while carrying out repair works, which it hopes to have finished in time for the Olympic Games.
"The agency took the decision to close the Boston Manor Viaduct because in entering the final stages of this complex work we found a further crack in a highly stressed zone over the bridge supports," the Highways Agency said in a statement.
It added that the problem requires a "unique repair solution" and that the road will remain closed until the work, which is being conducted from below the structure, has been completed.
As the M4 is a major entryway to London, it will be vital for the repairs to be completed in time to accommodate the extra traffic expected during the Games.
Author: Editor Gavin Pearson (firstname.lastname@example.org or 0207 202 0255)