In what is techinically known as a “quench” the £4.4billion collider is said to have lost up to a tonne of liquid helium after some of its superconducting magnets inadvertently heated up this morning.
Firefighters were said to have been dispatched to that area of the tunnel and the vacuum in that of the beam pipe was lost.
The problem means it will be impossible to stage its first trial collisions next week, and further delays could follow once the damage has been fully assessed.
It could even take several weeks to resolve, depending on the extent of the damage.
Engineers were still investigating the extent of the malfunction this 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.
The fault does not pose any longer-term threat to the LHC.
James Gillies, head of communications at Cern, said: “The incident occurred while we were commissioning the final sector, and a lot of helium has leaked into the tunnel. We are investigating now, and we should have a clearer picture over the weekend.
“How long it takes to resolve depends on what it is. It could be very little time, or it could be many weeks. I don’t want to speculate until we have more information. It certainly means we will not have collisions on Monday, or indeed next week.”