The M4 motorway has been closed in both directions between junctions 1 and 3 in west London and is expected to remain closed for up to five days, the Highways Agency said.
The section of motorway was first closed at 8pm on Friday.
The closure is part of continuing work being carried out to the Boston Manor Viaduct, just west of junction 2.
The Highways Agency said it decided to keep the Boston Manor Viaduct closed because during works it found a crack in a "sensitive location", which requires the viaduct to be kept closed until it is repaired.
The agency said it expects work to be completed in time for the Olympic Games later this month.
A signed diversion route is in place on adjacent dual carriageways, the A312 and the A4.
Drivers are being advised to keep up to date with traffic conditions and allow extra time for their journeys.