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 closure is part of continuing work being carried out to the Boston Manor Viaduct, just west of junction 2.
The Highways Agency says it has 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.
But there are fears of Olympic road chaos as the route is the main motorway from Heathrow to London.
The M4, part of the Olympic Route Network, will be vital for transporting visitors into the city for the Games later this month and it would be a major embarrassment for organisers if the restrictions can't be lifted in time.
These motorists aren't happy about the long delays: