Greenpeace activists who occupied a waste incinerator were cleared yesterday of causing £10,000 of damage after a jury agreed their protest was intended to prevent the plant harming people's health.

The jury at Wood Green Crown Court, north London, cleared five activists of causing criminal damage to the incinerator in nearby Edmonton when they camped in the 100m-tall chimney, shutting it down for six days last October.

Lawyers for Greenpeace had argued the protest was not illegal because the protesters had "lawful excuse": their action was taken to prevent the incinerator harming people or other property. This was the second time such a defence has won a high-profile case for Greenpeace. Last September it led to the acquittal of 28 activists charged with criminal damage for attacking genetically modified crops.

After yesterday's result Greenpeace said the acquittals were not the result of "clever lawyering" but showed it was picking legitimate targets and had public support.

The activists ­ Robert Gueterbock, 30, of Balham, London; Frank Hewetson, 36, of Queens Park, London; Richard Watson, 36, of east London; Christian Aslund, 26, from Norway; and Janet Miller, 45, of Buxton, Derbyshire ­ all denied one count of damaging property. They had climbed the chimney and unfurled a banner branding it a "cancer factory". The criminal damage charges related to holes drilled in the chimney to hang the banner.

Christopher Ball, for the prosecution, dismissed the protest as a publicity stunt and said the incinerator was no "risk to life or limb".

He said the plant was essential to help dispose of rubbish and produced electricity as a by-product.

The activists had said the incinerator, owned by London Waste Limited, had broken pollution limits "on many occasions" so they had every right to believe it was breaking the law at the time of the protest.

After yesterday's verdict, Mr Gueterbock said: "This verdict sends a very clear signal to the Government that they are going to have to rethink their plans to increase the number of incinerators from about 13 to 165. People do not want them."

Blake Lee-Harwood, head of the toxics campaign at Greenpeace, said the verdict showed its evidence was correct ­ the incinerator increased the risk of cancer or breathing difficulties and could contaminate produce grown in the area.

"The lawful excuse defence worked but that does not mean we can use it every time we get into trouble with the law. It only worked this time because it was true," he said.

Paul Egan, a spokesman for London Waste, said the protesters had been "naïve in the extreme".

"Not only did their actions damage the environment and disrupt residential waste collections but, as we also deal with clinical waste, they also came close to provoking a crisis in some of London's major hospitals."

He added that the shutdown cost £600,000, half of which had to be met by council tax payers in north London.