Highways Agency bosses are now hoping to have the stretch west of Junction 2 fixed and open to traffic “within the next few days”.
Repair work is being overseen 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 contractors were “working round the clock”.
It added: “Due to the complexity of the operation, more time than originally envisaged is required to complete the work safely.”
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.”
Cracks in the viaduct were discovered during a planned routine investigation of the structure in March and repair work has been ongoing.
During the final stages of the complex repair work the Highways Agency found a new crack in a highly stressed part of the bridge supports.
This necessitated a unique repair solution which involves manufacturing bespoke, specialist steel plates to install over the crack.