The costly blunder caused transport chaos in the capital and left engineers battling against time to clear up the mess before the concrete set.
Contractors working for the Bam Nuttall/Taylor Woodrow joint venture on the £700m Victoria Station upgrade were pouring the concrete to fill voids in new excavations.
But the concrete found its way into a signal room inundating the vital equipment controlling one of London’s busiest underground lines.
A source said: “We understand that a foot of concrete burst into the control room, having been poured into an escalator void.”
Nigel Holness, London Underground’s operations director, said: “Our contractors were working on the new station in an area next to the Victoria line signal control room.
“These works involved the use of water and cement which leaked into the room, damaging equipment. “This has meant there are no signals working on the southern section of the line.
“Our engineers are working hard to resolve the situation as soon as possible to get services back up and running, but the line is expected to be affected for the remainder of the day while repairs are carried out.”
The site team worked through the night to fix the problem and the line reopened Friday morning.