At regular intervals over the past few years, Tony Blair has given strong speeches on the importance and urgency of tackling climate change. He has told us that this is the single greatest challenge facing the international community, and that the scientific evidence is alarming. He is certainly right about that. He has also said he is personally passionate about solving the problem.

Greenpeace has been sharply critical of Mr Blair on other issues - on GM, nuclear power and, above all, on Iraq. But on climate we have tried to believe in his sincerity. We need politicians to take the lead, and we need to support them when they do. It's not our style to ask automatically, as Jeremy Paxman does: "Why is this bastard lying to me?"

On the diplomatic stage, Mr Blair has done quite well. He lobbied Russia to ratify Kyoto, and has said that climate will be a priority for his G8 presidency next year. But as he admitted in his most recent speech, in September, he will not be taken seriously on the world stage unless he has delivered real reductions in emissions at home. On this, his record is pathetic.

The Government has failed - and now seems to have given up - on controlling emissions from transport. Mr Blair promised fuel protesters in 2000 that he would give them cheap fuel just as soon as the Government could do without the revenue. On emissions from air transport, the fastest-growing source, the Government has not even tried, but instead favours uncontrolled expansion and new runways all round. Combined-heat-and-power plants, which dramatically increase efficiency and reduce emissions, have been starved of support and sacrificed to the objective of liberalising the energy market, so the CHP sector is now in decline.

The Government can boast a reasonably good record on promoting renewable energy, particularly offshore wind, and it did introduce the climate change levy back in 1999. But these have not been enough to make up for the numerous retreats, and the net result is that emissions of carbon dioxide have increased since 1997. So much for leadership.

Through all this, he kept on giving the speeches, and we kept on trying to believe him - perhaps for too long. But since his September speeches two further failures have convinced us we cannot trust him on climate. First, the Government caved into pressure from the CBI and increased the amount of carbon that industry will be allowed to emit under the new European emissions trading scheme which starts next year. The CBI is notorious for resisting any progressive policy and exaggerating the impact of proposed environmental changes. Even its former director Adair Turner has said that claims that environmental policy damages competitiveness are bogus. But still the Government, at No 10's insistence, capitulated and will allow industry to emit as much carbon in future as it has in the past.

Hot on the heels of this climbdown came the Housing Bill, in which the Government rejected amendments supported by Conservatives, Liberal Democrats and several Labour backbenchers, which would have increased the energy efficiency of the housing stock and increased the efficiency standards of social housing. On top of the climate benefits, this would have helped save some of the thousands of people who freeze to death every year in Britain. But the Government said it would be too expensive. (Following pressure from several backbenchers, the Government did eventually accept the amendment on overall energy efficiency, but persisted in rejecting its responsibility to those in social housing.)

So Mr Blair cannot be trusted to resist industry lobbying. He cannot be trusted to stand up to the motoring lobby, the airlines, or even the Treasury. Fancy speeches are not enough - tackling climate change requires radical action. All the evidence suggests that Mr Blair will not provide it. We intend to do all we can to expose the dangerous gap between rhetoric and reality.

Stephen Tindale is executive director of Greenpeace