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Astronomers have spotted the earliest known spiral galaxy, dating to just three billion years after the Big Bang.

Theories of galaxy formation held that the Universe was still too chaotic a place to allow such a perfectly formed or "grand-design" spiral to form.

It should take far longer for gravity to bring matter into thin, neat discs.

But a team reporting in Nature says the galaxy BX442 got the gravitational "kick" it needed to form a spiral from a smaller "dwarf galaxy" orbiting it.

They first spotted BX442 as the one and only spiral-looking object in a survey of 300 galaxies carried out by the Hubble space telescope, when they were shocked to see what looked to be a spiral galaxy.

"What we've learned when we look at galaxies at that epoch is that they're very dynamically hot," explained lead author of the study David Law from the University of Toronto's Dunlap Institute for Astronomy and Astrophysics.

"Even though we see some discs existing at that time, they're very thick and puffy, whereas the Milky Way has an... amount of random motion only about a tenth or so the amount of ordered rotation, giving rise to a very thin disc," he told BBC News.