Liberal Democrat leader Charles Kennedy has accused the prime minister of failing to take the "tough decisions" to tackle climate change.
The environment is Tony Blair's blind spot, said Mr Kennedy
He was reacting to Tony Blair's claim, to an MPs' committee, that the issue is the biggest facing the world.
Mr Kennedy said the PM had expressed concern over carbon emissions from the transport sector on the same day a huge road expansion project was announced.
Mr Blair insists Britain will meet its targets for cutting CO2 emissions.
But Mr Kennedy said the environment was the prime minister's "blind spot".
"He has no answer to how, having only cut CO2 emissions by 0.2% since he came to power, he will meet his target to cut it a further 12% by 2010," argued Mr Kennedy.
"The prime minister clearly doesn't regard the environment as a priority and he won't take the tough decisions to address the environmental problems we all face."
Huge road expansion projects, like the planned M6 toll road, were not the answer to long-term congestion, Mr Kennedy added.
These criticisms were backed by green group Friends of the Earth whose campaign director, Mike Childs, said allowing industry to increase emissions, build new roads and expand air travel was not "the way forward".
"The prime minister may be genuinely concerned about climate change, but unless his government starts delivering policies that will tackle the problem, people will doubt his sincerity," Mr Childs said.
"Unless Tony Blair and his ministers put the long-term future of the planet ahead of short-termism the situation will only get worse."
Earlier, Mr Blair told a committee of senior MPs the international community must do more to tackle environmental damage.
He said pressure was being maintained on the US, who opted out of the Kyoto protocol to cut carbon emissions.
Mr Blair also indicated that the "door would not be closed" on a revival of nuclear power as a source of clean, green energy.
"It is a question of balancing the cost and making sure the concerns people have about safety are dealt with," he added.
But chief executive of the Energy Saving Trust Philip Sellwood said uncertainties about dealing with nuclear waste meant energy efficiency and renewable technologies were the best solution to environmental problems.