Around 50,000 customers were left without broadband connections at the weekend after cables were damaged in an underground tunnel in east London.
Both business and residential users experienced problems with landlines and internet connectivity after a third-party contractor caused "significant damage" to BT lines.
The tunnel is 32 meters below street level and was blocked completely following the incident on Saturday afternoon. BT confirmed that the cables will need to be brought to the surface then re-routed as part of the extensive repair work.
A statement released by BT Openreach explained that due to the serious nature of the damage it "may be some days" before services return to normal.
However, work to clear the tunnel started on Monday with BT hoping to gain access to the cables and solve any remaining problems as soon as possible.
The majority of residential landlines have now been reconnected and engineers have restored services by "using alternative methods of connection".
"We would like to apologise to all impacted customers," the broadband provider added.