Emergency repairs are now being carried out on the Boston Manor Viaduct and the motorway was closed between Junctions 1 and 3 on Friday for “up to five days” according to the Highways Agency.
Measures were introduced Sunday night to allow the motorway to reopen between Junctions 1 and 2.
Repair work is being caried-out by the Connect Plus consortium of Balfour Beatty, Atkins, Skanska and Egis Road Operation under its Area 5 contract with the Highways Agency.
The agency said workers had “found a further crack in a sensitive location” on the viaduct just west of Junction 2 of the M4.
Large vehicles have been banned from the viaduct since March when hairline cracks were discovered in steel beams.
An agency statement said: “We took the decision to close the Boston Manor Viaduct because in entering the final stages of this complex work we found a further crack in a highly stressed zone over the bridge supports.
“This requires a unique repair solution, which we are currently designing, which requires us to keep the viaduct closed until the repair is complete.
“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.”