A senior Jubilee Line source said it was "very questionable" whether the North Greenwich to Stratford section would meet its revised spring opening date following the repeated failure of the line's communication system during tests.
Project insiders said the JLE's computerised integrated communication system, which includes signalling, safety and security systems, is "overloading" and "crashing" in tests.
Inspectors from London Underground Chief Engineers Group are to start witnessing tests on the system next week. They have to certify the line's safety and sources said there was "every chance" they will refuse to sign a completion notice.
Electronics group GPT, now trading as Marconi Communications, is running the £61m Contract 204 to install the communications system. It refused to comment.
Fault at the heart of system
The problem is said to be in the communication system's central control unit, which collects and distributes signals and communications on the line. It also handles the line's public-address system and visual display services.
A source said: "The central control can only operate with small bits of information going through. When the system has to carry a lot of information, the whole thing crashes." Another project source said: "It could be a single terminal causing the problem, but that could take weeks to find." A JLE insider added that if the problem was not solved, London Underground could be reduced to operating the system manually.
"In theory, the system could be operated manually. But it would be very labour intensive, and they would have problems getting enough suitably trained people."
Not the first glitch
Last year, GPT experienced problems during the installation of the line's supervisory control and data acquisition system. The package was eventually split between GPT and other JLE contractors.
A Jubilee Line spokesperson would not say whether the system had crashed during tests, but in a written statement said: "The communications systems are not crashing all the time.
"Work is progressing well and it will not delay trial operations or the opening of the first phase of the railway.
"London Underground, Marconi and the software suppliers are working together on the commissioning of these systems.
"There are no problems with functionality, but each station has a large amount of data, and it is only when it is finally integrated with the rest of the system that some problems can be seen and dealt with.
"This is all part of the testing procedure and it is normal when checking software to fix data on site." US troubleshooter Bechtel, which stands to lose part of its £12m "success fee" if the line fails to meet its deadline, refused to comment.
The situation has echoes of an earlier difficulty, when London Underground installed the untried "moving block" signalling system on the JLE.
After a number of problems, London Underground eventually scrapped "moving block" and installed a traditional, "fixed-block" system instead.
