Grey shadow/spot appeared on my camera?

I have just noticed a largeish, grey translucent spot has appeared on the camera of my iPhone 5s and it appears in all of the photos. I have tried cleaning the lens with a soft cloth and restarting my phone and resetting my preferences but it is still there. Does anyone have any idea what it could be or how it could be fixed?

It is at the top in the middle (slightly to the right) in this image.

Thanks in advance.