Picture: The ghostly Angel of the Vatican

As the Pope made an address nearby, he decided to capture the stunning image on his camera.

But it was only when Mr Key, 48, and his wife Susan, 44, returned home and and downloaded their photographs that they noticed a strange apparition in the picture.

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An apparition? Andy Key's image of a 'guardian angel' above the heads of other visitors to St Peter's Basilica

They were amazed to see what looked like the image of a guardian angel above the heads of other visitors to St Peter's Basilica.

Mr Key, from March, Cambridgeshire, said: "It looks like an angel hovering on the people's heads.

"No-one can explain it - there's nothing on their heads for the light to bounce off."

Mr Key went on the holiday to mark his retirement from his job as a police constable for Cambridgeshire Police.

He and his wife, a medical secretary, were looking around the famous church when they noticed a large group of people and realised the Pope was making an address nearby.

"I was taking pictures of all the things around us and saw this huge window with the light streaming through and thought it looked beautiful," he explained.

"I snapped the picture and didn't noticed anything until I got home.

"It looks almost like a hologram. I thought it was really spooky."

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Professional photographers have studied Mr Key's photo and are at a loss to explain what may have caused the image.

It was only revealed when he got home, plugged his digital Kodak camera into his computer and downloaded the images.

Mr Key, who now works as a student mentor at the Neale-Wade Community College in March, said: "It wasn't as if we had visited Rome for any particularly religious reason. We were simply sight-seeing as you do.

"I had not really thought about angels and stuff before. I don't know if I believe in it all but it does look like an angel.

"Several people have looked at it including a professional photographer and they can't work out what it might be - maybe it is a guardian angel."

It is by no means the first time holy images have unexpectedly appeared.

Two years ago, church organist Rita Clayson found a picture of Jesus Christ in an ancient tree in a small Northamptonshire village.

Twisted ivy roots helped to form his face on a tree trunk in a small spinney on the edge of Little Houghton.

Earlier this month, pregnant Amanda Skelding, 23, from Glasgow, saw what appeared to be a traditional image of Jesus in the grainy black and white picture she was handed after her ultraound scan.