Ads that claim products are carbon neutral using offsets are to be banned by Britain's advertising watchdog unless companies can prove they actually work, the Guardian can reveal, as Gucci becomes the latest company to wrestle with a high-profile green commitment based on offset . The Advertising Standards Authority (ASA) is to begin tougher enforcement of the use of terms such as "carbon neutral", "net zero" and "nature positive" amid growing concerns that companies are misleading consumers about the environmental impact of their products. part of a crackdown on greenwashing later this year following a six-month review. Under the plans, the ASA will take action against firms that tell consumers they can buy their products without worsening global warming or loss of nature on the basis of purchase offsets - unless they can prove they are actually effective. The move follows the ASA's recent enforcement of rights against Lufthansa companies a Etihad on ecological claims. Companies will still be able to talk about their sustainability efforts, the Guardian understands. (Patrick Greenfield)