A UFO clearly.

The camera automatically fired its flash in the relatively dark sunset and that flash is reflecting off an insect flying near your camera. The slow shutter speed for dark conditions and the blurring caused by the insect being too close to the lens gives the effect of an impressive streak. The surrounding visual clues (clouds, the landscape) lead you to believe it's something very large and a long way off. 

The object is an out of focus droplet of water, probably rain, brightly illuminated by the flash. If you take a photo in the dark of a subject that is far away, your camera flash will automatically carry on for as long as it can, to try to illuminate the scene properly.

The flash intensity does not stay constant, however. Instead, it falls away gradually, because it is draining a capacitor. The brilliant initial light, followed by the gradual fading, explains the way the tail diminishes in intensity as the drop falls.

In addition, the flash goes on for longer than average. This explains why the droplet, which would normally be 'frozen' by the flash as a bright circle, can move far enough to be blurred. The tails in such photos usually point downwards but sometimes are diagonal, depending on the wind.

This looks like lens flare caused by light from a source outside the field of view entering the lens at a sharp angle. It can be avoided by using a better lenshood.

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I am really sorry about this but after looking at the photo in Newscientist and the limited digital one on this website I can only conclude that the phenomenon shown here is Photoshop. A larger and less compressed photo might show details that validate this as some other effect, but currently I can't draw any other conclusions.