Subatomic particles were again clocked travelling faster than light by European researchers after the experiment was revised to rule out a certain type of error.

"The positive outcome of the test makes us more confident in the result," said Fernando Ferroni, president of Italy's National Institute of Nuclear Phyiscs (INFN), in a statement releasing the new results Friday.

But he cautioned that the result can only be confirmed with great certainty if similar measurements are performed elsewhere in the world.

The OPERA detector in Gran Sasso, Italy, is designed to detect neutrinos generated from protons 730 kilometres away at CERN, near the French-Swiss border. OPERAIn September, scientists at the European Organization for Nuclear Research, also known as CERN, announced that an international physics collaboration called OPERA had clocked particles called neutrinos travelling faster than light — something that is impossible according to Einstein's 1905 special theory of relativity, which provides the basis for most of modern physics.

The neutrinos were timed between CERN's facility near Geneva at the French-Swiss border and the INFN laboratory in Gran Sasso.

The scientists said they were surprised by the finding, but were unable to trace it to an error, so they were asking others to scrutinize it carefully.

'The search is not over'

In response to suggestions from other scientists, they ran the experiment again with much shorter, widely spaced pulses of neutrinos, allowing them to match the neutrino detection at Gran Sasso with the original pulse at CERN far more precisely instead of relying on averages.

"One of the eventual systematic errors is now out of the way," said Jacques Martino, director of the National Institute of Nuclear and Particle Physics at France's national centre for scientific research, in a statement, " but the search is not over. There are more checks of systematics currently under discussion."

For example, he said, the synchronization of the time reference at the starting and end points will be done using a different method.

The new results have been published online on the ArXiv online physics archive and have been submitted for publication in the Journal of High Energy Physics. They have not yet been peer-reviewed.

The scientists involved are also still asking other researchers to independently verify the measurements at a different experimental site.

Their original announcement was greeted by skepticism from many physicists, who said a very high standard of proof is needed for such an extraordinary result, which was more likely the result of an undiscovered error.