ONE of Britain's most ­prominent cosmologists has reignited the debate over the presence of alien life in our galaxy - by claiming the human race is alone.

Professor Brian Cox has argued that as there is no evidence of alien intelligent life ever having existed at any stage in the history of the galaxy, the natural conclusion is that no such race has ever existed.

He believes any civilisation would have explored the galaxy by now, leaving behind traces of their existence in the process.

Prof Cox said: "(The absence of alien artefacts) suggests that there is only one advanced technological civilisation in this galaxy and there has only ever been one, and that is us.

"If we are the only ­intelligent civilisation in the galaxy, that makes us indescribably precious and valuable. We are the only island of meaning in an infinite sea of stars, and that confers a responsibility to survive and explore the stars."

The Milky Way contains up to 400 billion stars. But there are thought to be around 100 billion galaxies in the observable universe.