Officials have shut the main highway linking London and Heathrow Airport for emergency repairs, less than three weeks before tens of thousands of extra visitors descend on the city for the Olympic Games.
The Highways Agency said Saturday that a west London stretch of the M4 could be closed for five days.
The motorway was closed late Friday after workers doing roadworks found a crack in a "sensitive location" on a viaduct.
The agency says it expects repairs to be completed in time for the Olympic Games, which begin July 27.
Heathrow will be the main port of entry for tens of thousands of athletes, officials and journalists attending the games.
London Mayor Boris Johnson says it is vital the repairs are completed "as speedily as possible."