Environmental charity WWF UK has severely criticised the government for what it says is a lack of action on climate change and the environment.
Protesters blocked the rear entrance of Downing Street
It says the UK has wasted opportunities presented by its presidency of the EU and the G8 group of wealthy nations.
Meanwhile, Greenpeace has dumped coal at Downing Street as a protest at the government's environmental record.
But a Downing Street spokesman said the government had achieved wide consensus in the G8 on tackling greenhouse gases.
A truck bearing the slogan "Blair - Climate Failure" dumped several tonnes of coal across three entrances to Downing Street.
Another, heading for the Whitehall entrance, was stopped by police.
Greenpeace says Mr Blair is "rowing back" on his commitment to the Kyoto Protocol - the international and legally binding agreement to reduce greenhouse gas emissions worldwide.
Director Stephen Tindale, a former environment adviser to Labour, said: "So far all he has done is make speeches."
Last year, environmental charities applauded the prime minister when he said he would make climate change a priority.
But WWF UK now says he was trying to please environmentally concerned voters more than demonstrating the will to use leadership in tough negotiations.
Last week, Mr Blair said the "blunt truth about the politics of climate change" was that no country would want to sacrifice its economy to meet the challenge, although they all knew they must develop on a "sustainable basis".
The charity says he has undermined the most central plank of climate change policy by suggesting binding targets to cut pollution are incompatible with economic success.
Campaigns director Andrew Lee told BBC Radio 4's Today programme: "This is exactly the language we hear from George Bush.
Mr Blair was undermining the people "trying to take action on climate change" and "giving succour" to the opponents of the Kyoto treaty, Mr Lee added.
WWF UK is also highly critical of the government's handling of European legislation to restrict the use of certain chemicals.
Mr Blair's EU presidency "has to broker a final deal on a regulation that could protect millions of people and nature throughout the world", Mr Lee told Today.
But the Downing Street spokesman said the legislation remained part of the UK's work agenda.
Transport Secretary Alistair Darling told Today the government had not abandoned its targets to cut carbon dioxide emissions by 20% by 2010.
"Demanding and tough targets" were "absolutely necessary", he told the programme. "We need to take action now."
But Mr Lee said the government was "off-track" to meet its 2010 target, adding the UK's rising carbon dioxide emissions were "not a very good example of leadership".
The government's chief scientific advisor Sir David King agreed it was "quite likely" the government would miss its 2010 target - but it remained committed to cutting carbon dioxide emissions by 60% by 2050.
The UK's presidency of the EU and the G8 had achieved "an enormous amount", Sir David added.
"We have got a statement from the G8 that 'we will act with resolve and urgency now to meet our objectives of reducing greenhouse gas emissions'.
"All countries will need to accept tough targets - but at the same time the message needs to be got across that this is not at the expense of growing economies."
But Mr Tindale accused Mr Blair of "trying to kill off the Kyoto protocol".
"Blair is burning more coal than ever, our CO2 emissions have gone up, he is set to miss his own global warming targets," he said.
"We are facing a climate catastrophe - but Blair is back-tracking. He needs to act, and soon."