The striped Breton shirt as we know it today came into being shortly following the 27th March, 1858 Act of France which introduced the navy and white striped knitted shirt as the uniform for all French navy seaman. The official striped navy shirt or French sailor shirt soon became more generally a working mariner garment as it was picked up by many nautical men; seafairers and sailors across the region of Northern France. It was said that when a sailor had fallen into the sea, the distinctive block pattern of stripes on the French striped shirt made him easier to spot amidst the many blurred colors of the waves.
(Source: Williams, Mark. “Tails and the Unexpected”.)