The main motorway link from London to Heathrow Airport will not now reopen on Thursday as planned.
The Highways Agency said it had put back reopening junctions two and three.
The motorway was shut after a crack was found in a "sensitive area" of the road structure. Large vehicles were already banned from the elevated section, known as the Boston Manor Viaduct.
The Highways Agency has said it expects all work to be finished before the start of the Olympic Games.
It claimed that due to the complexity of the operation more time than originally envisaged is required to complete the work safely.
The agency said it still expected to reopen the road within the next few days.
Roads Minister Mike Penning said: "I share the frustration of motorists at the continued delay to the opening of the M4.
"However, let's be absolutely clear, safety must be paramount and I will not allow the road to be reopened until this repair work has been completed successfully.
"This is a very complex operation and it is not possible to predict precisely how long it will take."
He added: "I want to thank motorists for their patience and assure them that we are doing everything we possibly can to fix this problem and get the road safely reopened as soon as possible."