Plans for the Large Hadron Collider (LHC) to start smashing its first particles next week have been derailed after it developed a significant fault Friday.
The problem with one of the $7.2 billion accelerator's superconducting magnets means it will be impossible to stage its first trial collisions on Monday, and further delays could follow once the damage has been fully assessed.
While a faulty transformer that had hindered progress for much of the past week has now been replaced, the magnet failure is potentially more serious.
Supercooled helium that chills the LHC's magnets to 1.8 degrees centigrade above absolute zero was released into the accelerator's 17-mile tunnel in the incident.
Engineers were still investigating the extent of the problem Friday afternoon, and CERN officials could not say how long it would take to fix and what impact it would have on the LHC's schedule.
James Gillies, CERN's head of communications, said: "It certainly means we will not have collisions on Monday. We are now looking at the middle of next week at the earliest."