The most complex, "mind-boggling" crop circle ever to be seen in Britain has been discovered in a barley field in Wiltshire.

The formation, measuring 150ft in diameter, is apparently a coded image representing the first 10 digits, 3.141592654, of pi.

It is has appeared in a field near Barbury Castle, an iron-age hill fort above Wroughton, Wilts, and has been described by astrophysicists as "mind-boggling".

Michael Reed, an astrophysicist, said: "The tenth digit has even been correctly rounded up. The little dot near the centre is the decimal point.

"The code is based on 10 angular segments with the radial jumps being the indicator of each segment.

"Starting at the centre and counting the number of one-tenth segments in each section contained by the change in radius clearly shows the values of the first 10 digits in the value of pi."

Lucy Pringle, a researcher of crop formations, said: "This is an astounding development - it is a seminal event."

Mathematics codes and geometric patterns have long been an important factor in crop circle formations. One of the best known formations showed the image of a highly complex set of shapes known as The Julia Set, 12 years ago.