The aircraft was a single-seat, single-engined, ultralight monoplane. The mid-wing was supported by an over wing brace strut. It was being flown by an experienced ultralight pilot after having been purchased secondhand by another person. Following a normal takeoff the aircraft was observed to carry out a manoeuvre consistent with a deliberate stall and recovery. It was then seen to enter a level turn to the left, in which the angle of bank gradually increased to about 90 degrees, despite the aircraft being limited to an bank angle of 60 degrees. The turn continued through at least 360 degrees, during which the aircraft began to lose height. A witness said he then saw the aircraft roll out of the turn, still losing height, before it suddenly appeared to lose control and entered a steep spiral dive. The aircraft was seen to impact the ground heavily. An examination revealed that the welding of the attachment bracket, for the wing brace struts, had failed. This had allowed both wings to fold upwards and twist rearwards, resulting in an immediate loss of control. A laboratory evaluation of the failed weld indicated an extensive lack of fusion between the weld deposit and the bracket.