Neutrinos which appeared to have undermined a basic law of the universe by exceeding the speed of light might have done so even faster than first thought - or might not have done it at all, physicists in Italy said yesterday.

The scientists at the Gran Sasso laboratory said tests on the equipment used in their experiment had led to two question marks over its results, because of problems with possibly faulty cabling and with a timing mechanism.

The first "could have led to an underestimate of the time of flight of the neutrinos" while the second could have resulted in "an overestimate", said the Italian laboratory's OPERA group of neutrino specialists, relayed by the CERN particle physics research centre in Geneva, Switzerland.

The latest turn in the story - which set the scientific world in uproar when it broke last September - left the question of whether the neutrinos exceeded the speed of light unanswered.

More tests have been set for the coming weeks and months. CERN director-general Rolf Heuer said that the uncertainty was normal.

"Last September, the OPERA researchers said clearly the reading of the speed was an experimental result that had to be cross-checked," said Heuer, from whose centre the neutrinos are pumped underground to Italy.

"They have continued to check, and now they have found these two possible effects, one going in one direction and the other going in another," said the CERN chief.

Most scientists had been sceptical about the original measurements, which flew in the face of Albert Einstein's 1905 Special Theory of Relativity, which asserts that nothing in the universe can travel faster than light and underpins much of modern physics.

Edward Blucher, chairman of the department of physics at the University of Chicago, said at the time the finding would have been "breathtaking" if it were true.

The international team in Gran Sasso said it had run many tests over three years before going public with the finding.

A second experiment last year came up with the same result.

Other teams in the US and Japan working on neutrinos, invisible sub-atomic particles that pervade the universe but pass unhindered through matter, are preparing similar tests.

Dr Alfons Weber at the Rutherford Appleton Laboratory in Oxford, who is involved in both the US and Japanese efforts, said that measuring the speed of neutrinos was complicated and the OPERA team had done a careful job.