In a ruling published today, the regulator said that the Greenpeace UK fundraising website, www.greenpeacegiving.org.uk, included a page entitled "Redecorate a power station chimney".
The page said: "Chimneys, they're a bit dull aren't they? We prefer them when they have statements written down them, like ‘no new coal’ or ‘stupid’, which say what we think about them."
The page referenced the proposed Kingsnorth power station in Kent and included a picture of someone in a climbing harness painting on the side of a power station chimney.
"Direct actions are about being there in person to stop an environmental crime from taking place," the text said. "Shutting down dirty power stations is just one of the ways Greenpeace is working to secure a clean energy future, but painting down the side of giant chimneys cranks up the political pressure and throws a vital spotlight on one of the greatest threats to our climate".
The ruling says that Greenpeace responded to the claims by saying that when it took non-violent action it was with the intention of protecting the planet, and participants did not aim to break the law. It said that the article was tongue in cheek and that the activists who took action against Kingsnorth had been found not guilty of causing criminal damage.
The organisation said that it did not promote anti-social behaviour.
The ASA nevertheless upheld the complaint. It said that defacing property would generally be viewed as anti-social and would in some circumstances be illegal. The picture of a man painting slogans on the side of a chimney condoned such behaviour, the ASA said.
The regulator ruled that the claims must not appear in the same form. It also warned Greenpeace to make sure that its advertising did not encourage or condone anti-social behaviour in future.
A spokesman for Greenpeace said the organisation did not set out to break any law or regulation on its website so it had removed the advertisement.
He said the organisation would continue to "go to the front-line of environmental damage"."We’re going to keep doing that, whether it’s stopping climate change pollution at its source or sailing to the Arctic to stop oil drilling," he said.