Regulation 1007/2011 allows a manufacturing tolerance of 3% of total weight between the declared and the actual composition, but only for technically unavoidable deviations. You may not deliberately declare 100% cotton when the product contains 4% polyester. "100%" is only allowed for a single-fibre product, with small technically unavoidable traces.
If you sell knits with elastane, verify the real fibre content by test, because elastane is easy to underestimate. A wrong composition can lead to fines or withdrawal from the market.



