With roughly 65,000 delegates descending on Johannesburg later this month for the World Summit on Sustainable Development, the largest ever international conference on the environment, the absence of one British minister would seem likely to go unnoticed. Unless he is the environment minister, that is. Michael Meacher is, after all, the minister who in Britain has stayed on top of the brief since Labour came to power, and has won respect internationally and at home for his detailed grasp of the issues that Johannesburg will have to confront.

So far, Downing Street has merely insisted that the British delegation had to be reduced from a hundred to around 70, and confirmed that four cabinet-level ministers will attend. As we go to press, these are the Prime Minister, the Deputy Prime Minister, John Prescott (earlier rumoured to be on the exclusion list), who holds no environment or development brief, Mr Meacher's boss, Margaret Beckett, which has some logic, and Clare Short, who will cover the development side of the conference but is said to be rather lukewarm about doing so.

Mr Meacher appears to have fallen victim to Downing Street's judgement that there were brownie points to be gained by downsizing the British delegation. He was struck from the delegation after press snipes about junketing and gibes concerning the negative environmental impact of thousands of rich-country delegates flying in to famine-riven Africa. If the downsizing was meant to please the press, it has backfired. Friends of the Earth is now offering to pay the minister's fare and hotel bill, and both opposition parties are suggesting that the lack of detailed knowledge shared by the remaining delegates will make Britain a peripheral participant. It can hardly be ranked as a public relations success.

Downing Street is right in one respect only. What should be a ground-breaking conference, establishing an environmental blueprint for the 21st century, shows signs of becoming a three-ring circus. Originally intended as a forum for practical debate to follow up the successes (and failures) of Agenda 21 - the ten-year plan drawn up at the first Earth Summit in Rio de Janeiro in 1992 - this second Earth Summit has grown rapidly out of control. In addition to the 65,000 country delegates, their back-up staffs, teams from the various United Nations bodies and journalists (there are roughly 6,000 of those alone) who have already registered, a separate, parallel conference of lobbyists, business and specialist-interest groups such as environmental NGOs will attract no one knows how many more people. If ever a conference was destined to generate more heat than light, it is this.

None the less, the absence of Mr Meacher does raise fears about the continuing commitment of the British government towards global environmental issues, and, in particular, its enthusiasm for seizing the opportunity offered by Johannesburg to push forward the Rio agenda. However cynical we are about international summits, we should remember that two major international treaties evolved from the 1992 summit: the UN's climate change and biodiversity conventions both had their origins in Rio.

But to those who work closely with him, Tony Blair has made no secret of being thoroughly bored with environmental issues. He is unconvinced that human activity can exert an irreversible, negative effect on the earth's climate (the world's top scientists who sit on the Intergovernmental Panel on Climate Change disagree), and he is irritated by those whom he considers to be evangelistic, sandal-wearing purists. The company he keeps - "concerned" chief executives - reinforces his belief that business, given the facts, can be relied on to behave responsibly. Technological advance and men and women of goodwill will save the planet - not the green lobby.

Mr Meacher, on the other hand, is a committed environmentalist. His diligent mastery of the technical issues behind global warming and bio-engineering has earned the respect of the expert NGOs and fellow environment ministers around the world. He may even be a bit evangelical and wear sandals. Like his Tory predecessor John Gummer, Mr Meacher is a powerful advocate on green issues within government and, just as his predecessor got little backing from John Major, he is far "greener" than his prime minister.

But there is an important point at which Mr Meacher and Mr Blair make contact, and that is in their ambitions for the world's poorest people. Both believe that poverty, particularly in Africa, is a scourge on the civilised world. Both believe that access to clean drinking water and non-polluting energy are among the continent's urgent priorities.

The Johannesburg summit takes place as Africa feels the full impact of the rich world's broken promises given in Rio - most evident in the famine now blighting southern Africa, after two years of drought-induced crop failures.

The Prime Minister will be one of 106 heads of state at Johannesburg. The President of the United States may be the 107th. If he attends the summit, the nations of the world will certainly hope that the Prime Minister will stand with them in trying to persuade President Bush that America's reckless refusal to reduce carbon emissions and take seriously the targets set by the Kyoto Protocol has a real and unfolding impact on the poorest people in the world.

Johannesburg may turn out to be the biggest talking shop in history, but those who participate need to know what they are talking about. Mr Blair has the clout to achieve things in Johannesburg. His environment minister has the arguments and the expertise.

Mr Blair should not leave Mr Meacher at home.