The government's poor record on the environment in the UK will undermine its efforts to play a leading role at a key global summit on the issue, campaigners claim.
Friends of the Earth accused the coalition of supporting fossil fuel companies while failing to back renewables or protect nature and the countryside, putting "economic growth at all costs" above sustainable development.
The green group criticised the Prime Minister for failing to attend the Rio+20 summit, marking 20 years since the original Earth Summit held in Rio de Janeiro, Brazil, which will focus on sustainable development.
David Cameron was criticised by green groups for failing to attend the Rio 20+ summit
It called on Deputy Prime Minister Nick Clegg, who is attending the summit next week, to ensure it does not not undermine existing agreements on sustainable development and is not hijacked by those who want to maintain "business as usual".
Friends of the Earth also called for the government to take action at home, including by helping households struggling with high energy bills by developing clean energy sources in the UK and cutting energy waste.
Action is also needed to save British bees, protect nature, reduce consumption and put sustainable development at the heart of all polices from town planning to farming and the food system.
The call comes after a series of sombre warnings from the United Nations Environment Programme and scientists that the world was on track to hit "tipping points" that would cause major changes to the planet, seriously damage the natural resources humans depend on to survive.
Expectations that Rio+20 will take major steps forward to tackle environmental problems have been played down, but it is expected to launch a process to agree new global goals for sustainable development.
Environment Secretary Caroline Spelman told MPs that the Prime Minister's decision not to attend the conference did not mean that the UK was not taking it seriously.
Speaking to the Environmental Audit Committee before travelling to Rio de Janeiro, Mrs Spelman said David Cameron was attending the G20 meeting in Mexico and did not want to be out of the country too long amid the Eurozone crisis.
She insisted the UK was seen as a leading country in the run up to the conference, and had worked hard to build bridges between developed and developing countries.
But Friends of the Earth's director of policy and campaigns Craig Bennett said: "The coalition's poor record on protecting the environment at home will undermine UK credibility at these crucial talks to safeguard the planet.
"Its old-fashioned economic thinking sees it handing huge subsidies to oil and gas companies while breaking promises to protect nature.
"David Cameron promised to lead 'the greenest Government ever' but he can't even be bothered to go to the biggest international environmental summit in years.
"If he wants to show leadership on the world stage, he must begin at home - this means action such as championing clean British energy and developing an urgent plan to save our bees and protect our food."
And he warned that unless rich nations seized the initiative and pledged to take urgent action the summit would be "little more than a talking shop while the world hurtles towards destruction".