Australian physicists propose that it may have slowed over the course of billions of years.

It's entirely possible that the speed of light would have got greater and greater as you go back towards the Big Bang

Paul Davies, theoretical physicist

If true, it would mean a rethink of Einstein's theory of relativity.

The idea is floated in a brief communication in the journal Nature.

It is based on astronomical data involving light from a quasar, a very distant star-like object.

Observations suggest the light has taken about 10 billion years to reach the Earth.

What is more, a key constant involving the interaction of light photons and electron particles seems to have changed.

It appears to have been smaller 10 billion years ago.

According to Paul Davies, a physicist at Macquarie University, Sydney, this can be explained only if the speed of light or electron charge has changed since then.

"But two of the cherished laws of the Universe are the law that electron charge shall not change and that the speed of light shall not change, so whichever way you look at it we're in trouble," he says.

Star Trek hope

Studies on black holes suggest that the second option is more likely, according to Davies' team.

The theoretical physicist believes the speed of light was faster six to 10 billion years ago than its current value - 300,000 km (186,300 miles) per second.

"It's entirely possible that the speed of light would have got greater and greater as you go back (through time) towards the Big Bang and if so it could explain some of the great mysteries of cosmology," he says.

He admits that further work on light from quasars is needed to firm up the theory. In addition, the physics of black holes are known to be extremely shaky.

But there are startling implications if the law that nothing can go faster than light is overturned.

"Maybe it's possible to get around that restriction, in which case it would enthral Star Trek fans because at the moment even at the speed of light it would take 100,000 years to cross the galaxy," says Davies.

It's a bit of a bore really and if the speed of light limit could go, then who knows? All bets are off."