A Balfour Beatty and Carillion joint venture is contractor for the GE19 bridge, part the £363M main works contract for the East London Line extension.
Although no-one was injured in the accident, a serious train crash was narrowly avoided as the 84m span, 812t bridge dropped 20mm onto its permanent supports.
Packing supporting the bridge failed at one support point, causing a second temporary support to fail.
The bridge slipped to its final resting position at both points.
As the steel warren truss structure fell, one of the precast concrete slabs forming the bridge’s deck came loose and dropped onto railway lines below.
No trains were passing below the bridge at the time of the accident, but one train is reported to have hit the debris from the fallen concrete Omnia plank.
Network Rail said tens of thousands of passenger journeys were disrupted when lines leading east from Liverpool Street station were closed at 7.25pm on Wednesday 28 May.
GE19 was successfully jacked into place onto temporary supports during a Bank Holiday weekend possession at the beginning of May. The structure straddles Brick Lane and Network Rail lines at a 1:30 gradient. Work to complete the structure was ongoing when it fell from its temporary supports.
The plank that fell had been resting on the bridge’s steel girders awaiting a cast-insitu 250mm thick reinforced concrete base to be poured on top of it. Each plank is typically 60mm thick, and up to 3.75m long.
"A piece [of the bridge's temporary support] came off," said a Transport for London (TfL) spokesman.
No official explanation for this was available as NCE went to press.
Disruption to Network Rail's services prompted an angry response from the track operator.
Network Rail chief executive Iain Coucher wrote to TfL commissioner Peter Hendy, expressing, "concerns over TfL's handling and management of both the project and its contractors," according to a Network Rail statement.
"Network Rail has banned work on the bridge, effective immediately. This will remain in place until TfL has completed an investigation and reported back on the causes of the incident and what measures it will be putting in place to prevent a repetition," it reads.
The ban may cause delays to the project programme as construction of bridge GE19 is on the project's critical path.
Contractors plan to use it to transport rail equipment to the north of the line during the project's later stages.
"All the new track and rail systems are being done in a southerly direction from Whitechapel station," Balfour Beatty/Carillion East London Line construction manager Adam Stuart had told NCE in February.
"Once that is done they will move north from Whitechapel, and without GE19 in place we won’t be able to run the rail systems on the northern section of the line."
Network Rail also confirmed that it had closely examined all work sites where TfL was working adjacent to Network Rail property and has allowed work to go ahead at these locations.
TfL confirmed that the GE19 bridge structure was sound.
Balfour Beatty Carillion’s contract includes three bridge installations – GE19, another at New Cross Gate and one across Shoreditch High Street.
Balfour Beatty refused to comment on the bridge slip while investigations were underway.
NCE understands that TfL's investigation will take, "a couple of weeks" to complete.
The GE19 installation is the most complex project on the East London Line extension.
The old GE19 bridge, dating back to 1880, was demolished last Christmas.
The new bridge, designed by Scott Wilson and Benaim, is an 8m deep Warren truss, and was jacked into position onto a temporary structure designed by Fairfield Mabey during the 50-hour May bank holiday possession early last month.
A 300t temporary "nose" was attached to the structure to act as a counterweight to prevent the bridge falling onto the tracks during the launch.