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Subatomic particles called neutrinos cannot move faster than the speed of light, according to a new report.
The findings challenge a result reported in September that, if true, would undermine a century of physics.
The team at the INFN-Gran Sasso laboratory in Italy said they had measured faster-than-light speeds in neutrinos sent from Cern, 730km away.
Now a different team at the same lab reports findings that, they say, cast doubt on that surprising result.
The Icarus team at the Icarus experiment says that because the neutrinos sent from Cern do not appear to lose energy on their journey, they must not have exceeded the speed of light along the way.
The idea that nothing can move faster than the speed of light is a central tenet in modern physics, forming among many other things a critical part of Albert Einstein's theory of special relativity.
Critics have suggested from the start that the experiment by the Opera collaboration, who published the first striking results, must be flawed in some way.
One of the first objections to the experiment to be formally published appeared just five weeks later in the journal Physical Review Letters, co-authored by Nobel prize-winning physicist Sheldon Glashow.
Prof Glashow and his co-author Andrew Cohen argued that particles moving faster than light should emit further particles as they travel - in the process losing energy until they slow down to light-speed.
The Icarus team already had measurements of the spread of energies in neutrinos, detected in their underground instruments at Gran Sasso.
They showed in a paper again on the Arxiv repository that the neutrino energies they measure are consistent with slower-than-light-speed travel.
With the exception of Prof Glashow's theoretical paper, none of the results by the Opera or the Icarus team has been reviewed by the scientific community and formally published.
But the momentous nature of the finding has sparked a flurry of papers and ideas to challenge or support the idea that particles can travel faster than the speed of light.
It is clear the issue is unlikely to be conclusively resolved until other experiments around the world undertake similar measurements.
The Borexino experiment, also at Gran Sasso, the Minos experiment in the US and Japan's T2K facility are all expected to publish their results of similar neutrino experiments in the coming months.
By Jason Palmer Science and technology reporter, BBC News
The findings challenge a result reported in September that, if true, would undermine a century of physics.
The team at the INFN-Gran Sasso laboratory in Italy said they had measured faster-than-light speeds in neutrinos sent from Cern, 730km away.
Now a different team at the same lab reports findings that, they say, cast doubt on that surprising result.
The Icarus team at the Icarus experiment says that because the neutrinos sent from Cern do not appear to lose energy on their journey, they must not have exceeded the speed of light along the way.
The idea that nothing can move faster than the speed of light is a central tenet in modern physics, forming among many other things a critical part of Albert Einstein's theory of special relativity.
Critics have suggested from the start that the experiment by the Opera collaboration, who published the first striking results, must be flawed in some way.
One of the first objections to the experiment to be formally published appeared just five weeks later in the journal Physical Review Letters, co-authored by Nobel prize-winning physicist Sheldon Glashow.
Prof Glashow and his co-author Andrew Cohen argued that particles moving faster than light should emit further particles as they travel - in the process losing energy until they slow down to light-speed.
The Icarus team already had measurements of the spread of energies in neutrinos, detected in their underground instruments at Gran Sasso.
They showed in a paper again on the Arxiv repository that the neutrino energies they measure are consistent with slower-than-light-speed travel.
With the exception of Prof Glashow's theoretical paper, none of the results by the Opera or the Icarus team has been reviewed by the scientific community and formally published.
But the momentous nature of the finding has sparked a flurry of papers and ideas to challenge or support the idea that particles can travel faster than the speed of light.
It is clear the issue is unlikely to be conclusively resolved until other experiments around the world undertake similar measurements.
The Borexino experiment, also at Gran Sasso, the Minos experiment in the US and Japan's T2K facility are all expected to publish their results of similar neutrino experiments in the coming months.
By Jason Palmer Science and technology reporter, BBC News