The rehabilitation of the beleaguered Large Hadron Collider was on hold tonight after the failure of one of its powerful cooling units caused by an errant chunk of baguette.

The £4 billion particle-collider faced more than a year of delays after a helium leak stymied the project in its first few days of operation. It is gradually being switched back on over the coming months but suffered a new setback on Tuesday morning.

Scientists at the CERN particle physics laboratory in Geneva noticed that the system’s carefully monitored temperatures were creeping up.

Further investigation into the failure of a cryogenic cooling plant revealed an unusual impediment. A piece of crusty bread had paralysed a high voltage installation that should have been powering the cooling unit.

The cooling systems are in place to keeps the collider functioning at a temperature of 1.9 degrees above absolute zero. As soon as there is a small rise in temperature the super-conducting magnets stop functioning and fail safes come into operation to control the collider.

A spokeswoman for CERN confirmed that baguette was responsible for the latest hiatus, but she conceded that mystery surrounded the way it got into the vital power installation, which is protected by high security fences.

“Nobody knows how it got there,” she told The Times. “The best guess is that it was dropped by a bird, either that or it was thrown out of a passing aeroplane.”

“Obviously this was slightly surprising. Within the team there was some amusement once they had relaxed after initial concerns.”

The bread was discovered on a busbar - an electrical connection inside one of eight buildings above ground on the 17-mile (27km) circuit in the Swiss countryside.

The spokeswoman said: “The collider extends over a very large area – you have to have a very comprehensive system to try to avoid problems of this kind. We’re talking about a couple of days down time.”

Scientists hope that the temperature will be restored by around midnight tonight allowing work to continue. The failure of the cooler meant the temperature rose around 5 degrees to the equivalent of about -266C.

The first beams were injected into the LHC on September 10 last year, but nine days later a connection between two magnets failed. This caused a huge leak of the helium that cools the ring around which protons will be fired against one another at 99.9999991 per cent of the speed of light.

The leak inflicted further damage, and the accelerator was mothballed so that 53 magnets could be replaced. Engineers have since found and replaced other magnet connections that could have been at risk of causing a repeat of the fault, and installed other safety features to prevent another fault.

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