New figures from Mori Social Research found the number of petrol buyers who say they regularly use Esso pumps has fallen by about a quarter since the boycott was launched last year.
The boycott was launched after Esso parent ExxonMobil, the world's largest oil company, was accused of scuppering the Kyoto climate change talks. ExxonMobil also backed a campaign in the US last month calling on the Bush administration not to sign any new treaties on global warming at the Johannesburg environmental summit.
A letter sent out by a number of groups supported by the oil company said that access to clean drinking water was the "greatest environmental obstacle" and that global warming was the least important issue.
Celebrities including Bianca Jagger and Rory Bremner are backing the boycott campaign. Mori figures show that the number of British motorists who regularly buy petrol at Esso stations has fallen from 26% to 19% in 12 months, with 5% of motorists saying they were already backing the campaign and nearly half saying they would do so if asked by environmental groups.
ExxonMobil launched a court case against Greenpeace in June to stop the group using the company's logo in "Stop E$$o" publicity material and for damage to its brand. It won an injunction against Greenpeace in July after successfully arguing that the manipulation of Esso's logo made it resemble Nazi Germany's SS.
The court case was seen as a victory by the "Stop E$$o" campaign and a sign that the grassroots campaign was having some effect.
Greenpeace campaigner Rob Gueterbock said: "The chickens are coming home to roost for the world's number one climate villain. For years, Esso has sabotaged every meaningful effort to tackle global warming, including this week's attempt to strike a deal at Johannesburg."
The campaign in the UK is a coalition between Greenpeace, Friends of the Earth and People & Planet.
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