The UK’s advertising watchdog has written to 17 “well-known” fashion brands advising them to review how they market sustainability claims amid an ongoing crackdown on greenwashing.

The Competition and Markets Authority concluded an investigation into companies including Boohoo and Asos earlier this year, requiring the retailers to clarify their green claims. Its latest salvo of warnings came with the publication of a compliance guide designed to help the sector ensure its sustainability marketing fits within the requirements of the law.

The letters highlight areas of concern, focused on vague or general claims that could mislead consumers. Some examples include labelling products as “recycled” when only a small proportion of recycled materials was used to make them and marketing collections or ranges as in some way sustainable without clearly stating the criteria used for such a designation.

The CMA said it has not launched enforcement action against the brands it has cautioned and declined to name them.The warnings came with a sting: a reminder that incoming regulatory changes mean the CMA will soon have the authority to fine companies up to 10 percent of global revenue if they break consumer law.

“Whether one of the 17 that got a letter or not… every business in the fashion sector should take note of the guide,” the CMA’s interim senior director of consumer protection Hayley Fletcher said. “Now is the time for businesses to reflect and get it right.”

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