Professor Brian Cox comes up against one of the wonders of the universe even he cannot explain when he appears in the latest episode of Doctor Who.

The nation's most famous physicist is called upon to try and help explain what is going on when mysterious black cubes start to fall from the sky, but is left stumped by the weird events in the latest episode, called The Power of Three.

He joins the Timelord, played by Matt Smith, and a real lord - guest star Lord Sugar - in the episode.

Cox said: "I have been a Doctor Who fan for a long time - from Pyramids of Mars and The Hand of Fear onwards, with the great Tom Baker and Elisabeth Sladen. I am also a huge fan of Matt, so it was a tremendous honour to be part of one of the great institutions of British television."

Lord Sugar appears in a scene where he asks his apprentices to try and sell the mysterious objects - with pretty poor results.

He said: "I have watched Doctor Who for over 40 years and I was flattered when they approached me to make a cameo. It's an honour to be appearing in such an iconic programme, not to mention the street cred it gives me with the grandkids."