"This decision will transform the tea industry which has been suffering for many years from oversupply and underperformance," Unilever Chief Executive Patrick Cescau said in a release.

Rainforest Alliance certification requires three levels of sustainability - worker welfare, farm management and environmental protection. The first certified tea will come from Kericho, Kenya, an estate expected to be certified within weeks, to be sold in European restaurants and to caterers in August.

The tea will sell at a premium, about 10 to 15 percent higher than average auction prices, with the aim to help farmers receive higher prices and improve their standard of living, according to Rainforest, which has reached several such agreements in the coffee industry.

Unilever plans to have all the tea used in PG Tips and Lipton yellow Label tea bags sold in Western Europe certified by 2010, with all Lipton tea bags Rainforest Alliance certified by 2015, Rainforest spokeswoman Gretchen Ruethling said.

The European based Anglo-Dutch company said it currently buys about 12 percent of the world's black tea supply.

The company's other sources in Tanzania, Malawi, Indonesia, India, Argentina and Sri Lanka will follow suit, Rainforest said.

China is the world's biggest tea producer, followed by India, Kenya and Sri Lanka.

"The company plans to include their other European brands as soon as enough tea becomes available," Ruethling said in an e-mail.

In 2007, Rainforest partnered with other companies including McDonald's Corp.'s McDonald's UK, Whole Foods Market and Holiday Inn hotels for certified coffee.