The M4 motorway in west London has been reopened in both directions between junctions 1 and 2, the Highways Agency said.
The motorway, which is the main route from London to Heathrow Airport and the West of England, was closed between junctions 1 and 3 on Friday night for ongoing repair works to the Boston Manor Viaduct, just west of M4 junction 2.
The motorway remains shut in both directions between junctions 2 and 3, and is expected to be closed for the next three days.
Drivers travelling into west London via the M4 today are advised to seek alternative routes via the M3 or M40.
London-bound traffic will be able to join the M4 at junction 2 and traffic heading out of London can stay on until junction 2, although there is a lane two closure in this direction.
The motorway was initially closed because Highways Agency staff discovered a crack in a highly stressed zone over bridge supports.
A spokesman said: "We expect the ongoing programme of repairs will be completed to enable the viaduct to operate in its full capacity in time for the London 2012 Olympic Games later this month.
"The closure is a sensible precaution, given the bridge's importance, both for people driving to and from west London and for the Games.
"The works are being carried out underneath the structure so the scale of the work is not visible from the M4 or from the local road network."
Diversion routes are in place on the adjacent dual carriageways, the A312 and the A4.