Extensive investigation, assessment, and structural reinforcement since Friday mean the M1 is now able to carry traffic in both directions between junctions 1 and 4 as of just after 4pm yesterday.
Two lanes southbound were reopened at 4pm on Monday in time for the afternoon peak travel period, the Highways Agency said.
Mabey Hire Services and Jackson Civil Engineering are carrying out the emergency propping work, along with structural experts from Atkins.
Together with Balfour Beatty, Skanska and Egis Road Operation UK, Atkins is responsible for this section of the M1 under the Connect Plus consortium.
Two lanes were already open northbound − the first since Saturday evening, and the second since approximately 1pm on Sunday.
Construction work on the bridge support, and investigations to determine what remedial work needs to be carried out following the extensive fire damage on Friday, are ongoing.