The M4 motorway has been closed in both directions between junctions one and three in west London following the discovery of a crack in a viaduct.
Queues have been forming around the busy stretch of road which is the main link to Heathrow airport from the capital.
The works come just three weeks before the start of the London 2012 Olympics, with the section of the motorway expected to be shut for up to five days.
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.
The closure is part of continuing work being carried out to the Boston Manor Viaduct, just west of junction two.
The Highways Agency said it decided to keep the Boston Manor Viaduct closed while a crack in a "sensitive location" is repaired.
The agency said it expected the work to be completed in time for the Olympic Games which get under way on July 27 until September 9.