Editors must love Shell. Almost whatever I have read about climate change and the UN talks in Copenhagen in recent weeks, it has been flanked by the familiar Shell logo somewhere in the background.

From geeky titles like New Scientist to politico mags such as Prospect and New Statesman; and newspapers like the Guardian, the world's second largest corporation has been splashing out – filling screens and newsprint with adverts and underwriting special supplements. Shell also sponsored a major research project by the Economist Intelligence Unit, called Countdown to Copenhagen, launched early this year at a Shell-sponsored "sustainability summit".

Nobody is suggesting that Shell is writing the copy. And surely only the most craven editor would leave out criticism of oil companies like Shell. But the unmistakeable message is that Shell is going green.

It's not just a subliminal message, either. The ads are all about Shell developing new low-carbon technologies, like carbon-capture, biofuels and "helping our customers use energy more efficiently". They have pretty images, like a butterfly net catching CO2, and a pocket calculator with a button marked "less CO2".

It won't be easy, says the message: "We'll need to think the impossible is possible." Trouble is, in reality, Shell wants to think the possible is impossible. As its recently retired chief executive, Jeroen van der Veer, said earlier this year of wind, solar and hydrogen power: "I don't expect them to grow much at Shell from here."

Back then I wrote that "Shell is the new Exxon". But the latest evidence suggests it is worse than that. A new study of the environmental performance of the world's top 10 oil and gas companies by the Madrid-based environmental auditing company Management & Excellence puts Shell last of all the western majors. That's behind BP, Total, Chevron and even ExxonMobil.

Shell has fallen from fourth place to seventh in the past year, and is now propping up the bottom of the table with two Chinese oil giants, Sinopec and Petrochina, and the Russian monolith Gazprom. None are known for their environmental credentials.

The audit analyses the 10 companies according to 198 different criteria. Shell gets a rating of 51%, compared with top-ranking BP's 77% and Exxon's 62%.

Shell's new chief executive Peter Voser last week made one statistical claim for his company's progress to date. Its chemical plants were, he said, 8% more energy efficient that in 2001.

Good for them. But most other companies are doing better. The M&E study found Shell next to bottom on energy savings.

Shell failed to make the grade in other areas, too. It may spend millions promoting its expertise in alternative energy technologies, but Shell came in the bottom half here, too, with only half the scores of BP, Chevron and the Brazilian oil giant, Petrobras. Once, BP and Shell were bracketed together as companies taking the lead in expanding into renewables. But the report says that among the top 10 today "only BP seems to have a real business in alternative energies".

Shell spokesman Shaun Wiggins said: "While Shell is aware of Management & Excellence, we have made a conscious choice to not participate in its rankings survey process." The company says it prefers other environmental audits.

The findings will come as no surprise to those who read Friends of the Earth's June report on Shell's Big Dirty Secret, which charged the it with being "the world's most carbon intensive oil company".

Shell claims on its websites: "We were one of the first energy companies to acknowledge the threat of climate change." The tragedy is that this is true, but that so little has come of it.

I have lost count of the number of false dawns at Shell. At the Earth Summit in Rio in 1992, I reported Shell scientists promising that the company was going to plant tree across the tropics to soak up carbon dioxide. Whatever happened to that idea? Just before the Kyoto climate conference in 1997, Shell announced it was making a $500m investment in solar power. By the World Summit in Johannesburg in 2002 it claimed to be installing solar panels across the developing world. Today it is absent from that business too.

Wiggins said Shell has spent $1.7bn on renewable in the past five years, but now concentrates on biofuels because they are "closest to our core business". But he agreed that oil and gas still make up 95% of its business, and the truth is that the company has flattered to deceive for almost two decades now.

Readers of its current adverts are directed towards a zappy and visionary website devoted entirely to what might happen in the future. But the future has been a long time coming for Shell. And it seems ever further away.