Image caption Lifelong Doctor Who fan professor Brian Cox is called upon to help the Timelord

Professor Brian Cox came up against one of the wonders of the universe even he could not explain, when he appeared in the latest episode of Doctor Who.

The popular physicist and broadcaster was called upon to try and help when mysterious black cubes start to fall from the sky.

But he is left unable to answer the events in Saturday's episode of the BBC TV series, called The Power of Three.

Another cameo role was taken by entrepreneur Lord Sugar.

Prof 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 [Smith, who plays the lead role], so it was a tremendous honour to be part of one of the great institutions of British television."

Image caption Lord Sugar also makes a cameo appearance

Lord Sugar appears in a scene in the BBC Wales-produced series 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."