PG Tips will become the UK's biggest ethical food and drink brand following certification of one of its major tea plantations by the Rainforest Alliance, The Grocer can reveal.

From January all PG Tips packs will carry the Alliance logo as brand owner Unilever becomes the first major company to introduce sustainably certified tea.

Unilever asked the organisation - which sets standards for sustainability to conserve wildlife and promote the wellbeing of workers and communities - to begin auditing its tea suppliers earlier this year. First to gain certification was the company-owned tea estate in Kericho, Kenya, which supplies tea for PG Tips in the UK and Lipton Yellow Label in western Europe.

Not all tea leaves used in the two brands come from Kericho but both will still be able to carry the Rainforest Alliance logo as they contain more than 30% of certified tea from Kenya. The company aims to have all its tea suppliers certified by 2015.

PG Tips is the UK's top-selling tea brand, worth £126m, and has grown in value 3.4% over the past year. Unilever said the logo would strengthen the value of the brand and lead to a long-term boost in sales. "We believe we can strengthen our brand by telling consumers what happens on our plantations," said Richard Fairburn, manager of the company's Kenyan tea estate. "It won't lead to an immediate jump in sales but it will help the brand in the long term."

The company said it chose Rainforest Alliance certification because of its approach to environmental and social issues. "We looked at the three major schemes - Fairtrade, Utz and Rainforest Alliance," said Michiel Leijnse, global brand development manager for category beverages, "but we felt in the case of the tea industry and the problems faced that the Rainforest Alliance was going to provide us with the best solution. The approach is comprehensive and the organisation works with large and small producers."

Unilever already produces a number of ethical products, including a Fairtrade variant of its Ben & Jerry's brand .