Conservative London mayoral candidate Zac Goldsmith's campaign have claimed it's "not clear where Sadiq Khan really stands" on tackling extremism.
However, Politics.co.uk can reveal that Goldsmith himself has previously defended a series of extreme environmental groups.
While editor of the Ecologist magazine, Goldsmith wrote a piece titled "Who are the real terrorists?" in which he praised a series of so-called 'eco terrorist' groups who had destroyed genetically modified crops owned by agricultural corporations.
He suggested the action taken by these groups against the companies was justified however, because the firms threatened "the stability of the living world".
In 1998, Goldsmith wrote: "Angry at the prospect of giving in to corporate bullying, [these groups] are setting out to accomplish by 'direct action' what their political representatives have so lamentably failed to do on their behalf.
"From the Lincolnshire Loppers, who pulled up a demonstration crop of genetically engineered spring wheat, to the Kenilworth Croppers, who destroyed a display of GM wheat at the Royal Agricultural show; from the decontamination of an experimental crop of oilseed rape near Coventry to the destruction of a plot of AgrEvo's basra-resistant rapeseed in Australia by 'Mothers Against genetic Engineering'; from the decontamination of 30 tons of transgenic maize seeds in France by 120 members of the farmers' Confederation Paysanne, to mass gatherings outside Monsanto's headquarters in Missouri, the clear message is that 'normal' people are not prepared to allow their leaders to license away the stability of the living world".
Goldsmith also hit back at criticism from the press and politicians against the groups.
"Not surprisingly this demonstration of public resistance has generated a backlash from the mainstream. Congressman Bill McCollum for instance condemned direct action as 'terrorism in the name of mother nature', while Congressman Riggs described activists as 'terrorists engaged in a criminal conspiracy'. Some newspapers in England have complained that a number of campaigners were on government-funded educational grants. But to what greater use could students possibly put their grants than towards ensuring the world remain viable for future generations?"
He then praised the groups for their dedication to the cause.
"These dedicated people, from the old to the young, from mothers to grandmothers, from students to scientists, are referred to as 'hooligans', 'vandals', and 'terrorists'. But in the end we should stand back and ask ourselves honestly, 'who are the real terrorists?'"
Asked in a separate interview with the Guardian in 2000 about his career, he replied: "In ten years' time, I might be an eco terrorist."
Goldsmith attacked for 'anti-business' opinions
Goldsmith, whose campaign has suggested that his rival Khan would be a threat to the future of London businesses, also warned Ecologist readers that all governments were increasingly "controlled" by big business.
"Governments are to a worrying and increasing extent controlled by these corporations," he wrote, adding that: "Keeping big business happy is now one of the basic governmental priorities - both left and right - in every country of the world."
Former environment secretary Owen Paterson last year attacked 'eco-terrorist' groups for their "fanatical antagonism" which he claimed were "condemning millions to hunger" through their opposition to genetically modified crops.
However, Goldsmith has been a long-term opponent of certain big agricultural companies. In campaign leaflets sent out this month, Goldsmith highlighted his personal role in taking on controversial agrochemical company Monsanto while editor of the Ecologist.
"I ran the Ecologist Magazine for ten years and over that period I fought for the issues I believe in," he wrote.
"On one occasion, the magazine launched a campaign against Monsanto alleging undue influence over US regulators. They responded by waging a war against the magazine, forcing our printers to destroy our first edition and pressured newsagents to stop selling copies But instead of shutting us down, that edition sold over half a million copies."
Labour said today that Goldsmith's 'anti-business' opinions showed he was unfit to be mayor.
"Zac Goldsmith’s record of extreme anti-business politics shows why he's not fit to be Mayor of London, one of the world’s great commercial centres," Stephen Pound MP said.
"The major companies that are based here and trade globally are central to our prosperity.The fact that Goldsmith has compared legitimate businesses to 'terrorists' shows how out of touch with the real world he is. He is just not up to the job."
A spokesperson for Goldsmith today declined to comment on whether Zac still defended illegal direct action against corporations.
However, they defended Goldsmith's role at the magazine.
"Zac used The Ecologist to influence those in power - business and governments - to change the way they thought and their business practices, which has been to the benefit of people in the London, the UK and around the world," they said.
"After taking over a down and out magazine he turned it into a campaigning force fighting against corporate corruption and for consumers and the environment."