Pirelli has confirmed an 'external source' was the cause of Nico Rosberg's dramatic 180mph tyre failure during free practice for the Belgian Grand Prix.

Rosberg suffered a wild spin at Blanchimont after the right-rear tyre on his Mercedes exploded at high-speed, the German fortunate to stay out of the wall.

With replays suggesting some tread had sheared from the tyre as he rounded Les Combes earlier in the lap, Pirelli has now concluded that this led to the blow-out, but that it wasn't down to the 'structural integrity' of the tyre.

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Suggesting an 'external source' prompted the failure, Pirelli Motorsport Director Paul Hembery is certain this is the reason and there is no underlying tyre problem.

"We have conducted a thorough investigation to find out exactly what happened with Nico's tyre. This investigation now excludes any structural integrity issues. Based on the information and data available an external source of damage is the conclusion made. Quality data check on other tyres has shown no anomalies."