But moral certainty can be a double-edged sword, as Melchett discovered when he announced that he was joining the corporate social responsibility unit at Burson-Marsteller in January and the knives came out. Melchett was instantly recast as the green's Lord Haw-Haw, broadcasting the environmental movement's deepest secrets across the boardroom table. In the ensuing row, he resigned from the board of Greenpeace International.
How could you be 'uncompromising' and work for a network that has every ecologist's bad boy Monsanto on its global client list, asked the critics.
It was a 'staggeringly naive and stupid decision,' wrote George Monbiot in The Guardian: 'No-one threatens (the environmental movement's) future as much as the greens who have taken the company shilling.'
But Melchett believes that by working in a role that will allow him to counsel corporations, he is moving the relationship between NGOs and commerce into a more mature phase as the environmentalist movement evolves. He sees it changing from a young, small movement to 'becoming something substantial and significant, which can't be a sort of niche for odd individuals'.
The modern relationship between corporations and environmentalists, he argues, is far more complex and sophisticated than in the past.
'If a company wants to get out of a business that is damaging the environment, that's a positive,' he says. 'The reason why NGOs now adopt the approach of praising and attacking a company simultaneously is precisely because of that. You can welcome what BP Solar and Shell have done on the renewable areas as well as wanting faster movement on combating climate change. There's not only no contradiction but those two things are completely in tune with each other.'
Melchett remains very much the model of the modern environmentalist, cycling in to B-M's Bloomsbury offices. He has a long track record in the green movement. In addition to his time at Greenpeace, he has also worked for the World Wildlife fund, the Royal Society for the Protection of Cruelty to Birds and the Ramblers Association. From 1979 to 1981 he was Labour's spokesman on the environment.
He's self-deprecating, wondering aloud whether the CSR unit's clients will want to work with him, and adding that the job will be as much about him learning as it is about passing on his experience.
'If I have any value to people at B-M, it's precisely because I have particular experience and a point of view, which their experience and track record shows companies are interested in,' he says. 'I want mainly to reinforce the approach the CSR unit is taking to emphasise the importance of acting, of getting good environmental and social performance in place and being prepared to start from the position that action's going to be necessary.'
He cites the unit's reputation within the environmental movement as one of the reasons he was attracted to work for B-M. In particular with the Marine Stewardship Council - a joint venture between the WWF and Unilever - and forestry projects for B&Q, which are singled out during the course of the interview.
'B-M adopts the approach that companies need to act first, which will then help them build relationships with key stakeholders, NGOs and so on and that then builds reputation and builds business.'
Part of the attraction for Melchett appears to be the fact that corporations can make changes rapidly, put competitive pressure on rivals to act and even encourage governments to change the rules.
'In the end governments will have to take actions to set standards,' he says of two global environmental issues, deforestation and fishery depletion.
'But to get there we need far-sighted companies working with NGOs, which will show there's a way forward.'
His experience at Greenpeace shows that corporate leaders can set the agenda. 'Some of the most inspirational speeches about what we need to do to resolve the environmental crisis, in the time I was running Greenpeace, came from people like Bill Ford Jr or Ray Anderson of Interface, corporate leaders of American multinationals,' he says. '(BP chief executive) Sir John Browne's speech on climate change (at Stanford University, May 1997) did more to influence the debate in North America on climate change than any other individual.'
However, none of this new corporate awareness means the need for environmental protests has passed. 'These things happen, in part, because of the pressure that NGOs such as Greenpeace, Friends of the Earth, the World Wildlife Fund bring to bear,' he says. 'The need for that pressure is just as great as it ever was. This is not an either/or, in other words.'
In his new role, Melchett will act as an external adviser to the CSR unit for a few days a year as well as being available to do a limited amount of work for client projects. He will bring a 'reasonable understanding' of the issues and a knowledge of the way that NGOs frame issues and the values that underlie their approach to clients. 'A lot of this is about pulling back and distilling the essence, what sort of action will make a difference,' he says.
The argument is that some companies will be more willing to discuss their environmental issues at a meeting under the aegis of the CSR unit than they would with a pressure group. Besides which, says the head of B-M's CSR unit Richard Aylard, many companies wouldn't know how to start to build a relationship with an NGO.
For all the criticism of the B-M global client list past and present, some of whom are never going to find a place on Melchett's Christmas card list, he isn't the first environmentalist to find a home there. Des Wilson, (see p14) who worked for FoE, is a former B-M London deputy chairman and the current holder of that post, Gavin Grant, is an ex Body Shop head of comms. Simon Bryceson, a former FoE staffer, has also spent time with the company.
'I haven't changed my views about any of the issues and do not see any reason why I should or, indeed, any pressure or suggestion from anyone that I should,' affirms Melchett. He is unhappy with the claim that experts can't cross the line between industry, politics and NGOs. Not everyone is 'privileged or lucky enough to get a job at Greenpeace,' says Melchett.
'Greenpeace is never going to be successful and nor is any other NGO unless their beliefs are shared by millions.'
B-M makes much of its staff's ability to refuse to work for clients if they disagree with what they do. Melchett naturally confirms that he won't be working for Monsanto, which would clash with his other interests, such as his family's Norfolk farm, which is being converted to organic status, and his role as director at The Soil Association. In any case, the official line on the GM specialist is that it is 'a valuable and respected client for substantial parts of the B-M network round the world but we've never done any work for them in the UK'.
In general, says Melchett, his selections are likely to depend on what each project involves. It's a case of 'what action are we talking about, what can the company actually do', he says. 'After all, what environmentalists dream of is nuclear power companies turning to wind, agrochemical companies supporting organic farming, people who make PVC making biodegradable plant-based plastics.'
For many outsiders the PR industry is more about facade than real action and the fear has been expressed that Melchett would find himself used as a figleaf by the unscrupulous. 'It's a bit silly to think you can say: "I've talked to some individual and everything's going to be alright".
That's an absurd and childish approach, it's precisely the opposite of what attracted me,' he says. 'I'm not a PR expert but it's actually fairly self-evident that the days of comfortable facades are long passed.'
From his role outside of companies looking in while at Greenpeace, Melchett will now be effectively demanding change from within. The test will be whether or not the green ideal can translate into action at boardroom level by engaging with, rather than opposing, the enemy.