The critical letter was also delivered to all federal Labor members and senators just hours before the cabinet's climate change subcommittee was due to discuss the issue, which has become increasingly fraught inside the Government.

The letter was signed by environment leaders who have worked closely with the Rudd Government, including Greg Bourne from the World Wide Fund for Nature and Don Henry from the Australian Conservation Foundation, along with activist organisations such as Greenpeace and GetUp!.

The letter notes the "growing and disturbing speculation" that the Government's 2020 target to cut emissions will be in the range of 5pc to 15pc below 2000 levels.

This target is significantly below the 25pc to 40pc cuts suggested as necessary by the United Nations' scientific body for developed countries like Australia if the world is to avoid dangerous climate change.

The environment groups tell Mr Rudd in the letter that the 2020 targets his Government sets to cut greenhouse gas emissions "will be the defining test of your leadership".

It warns that without ambitious targets, Australia will not be able to play a leadership role at the UN climate negotiations.

The Climate Change Minister, Penny Wong, has been working to complete the targets for the emission cuts, which will be implemented from 2012.

Last week she reneged on a commitment to take the targets to the current round of UN climate talks in Poland and delayed their announcement until December 15.

As the UN climate talks opened in Poznan this week, officials from developing countries warned that richer countries must agree to make emission cuts of between 25pc to 40pc by 2020 if they want to secure a global climate agreement next year in Copenhagen.