he world’s largest public relations firm is ending its lucrative relationship with America’s powerful oil lobby – after more than a decade and at least $327m in billings.

Ties between the oil lobby and the PR firm ran deep.

Much of the advertising work for API was handled by an Edelman subsidiary, Blue Advertising.

The relationship was by no means exclusive. But Edelman had favoured status, according to the Climate Investigations Center, which has tracked the company’s complicated relationship with the fossil fuel industry. In 2008, the oil lobby paid Edelman $75m, more than a third of the $203m in revenues collected in membership dues from ExxonMobil, Chevron and other oil companies.

The lucrative relationship was not without costs. Over the past year, Edelman came under growing public pressure for its ties to fossil fuel companies and industry groups which have promoted misinformation about climate change.

Last year, Edelman was caught out when other major public relations firms announced they would no longer work for climate deniers, The company also faced scrutiny for advising TransCanada pipeline company to run a perpetual campaig against opponents of a pipeline project across eastern Canada. TransCanada later announced it had dropped Edelman.

Such hardball tactics – and the accusations of climate denial – put Edelman in an uncomfortable position with some of its other clients, according to Kert Davies of the Climate Investigations Center.

API does not explicitly deny climate change, but suggests there is some doubt whether burning of fossil fuels is warming the planet.