Environmental ratings are becoming big business as companies seek proof of sustainable and socially beneficial conduct, both to avoid accusations of greenwashing and convince investors and customers they have enduring value. Firms that issue ratings on environmental, social and governance (ESG) performance are set to be regulated in the EU and U.K.

Ruth Prickett graduated from Cambridge University with a BA hons in History and has specialized in business and finance journalism for the past 20 years. She was editor of Financial Management, the magazine...