BBC Environment Correspondent Margaret Gilmore reports

The bloom is the size of Cornwall and is visible from space as a white-coloured mass stretching from south Devon to the Scilly Isles and moving west.

It has been caused by a rapidly expanding microscopic plant, Emiliania huxleyi.

This sheds its chalky outer layers, colouring the sea turquoise or milky blue.

'Harmless but unusual'

Experts at Plymouth Marine Laboratory say it is harmless but unusually close to shore and was triggered by hot, calm weather.

Local fishermen say they have never seen anything like it.

One skipper, Douglas Pooley, said: "It was like something out of a science fiction movie.

"We steamed about eight miles and we were in amongst it all the time".

Another man who steered his boat through the bloom said: "It's like sailing through milk."