Australian Prime Minister Kevin Rudd has told Danish PM Lars Lokke Rasmussen he fears the Copenhagen climate change conference in December will not be a success.
The ABC reports the conversation was taped by a cameraman as the two discussed the issue of climate change on the sidelines of the G8 conference in Italy.
Mr Rudd told Mr Rasmussen that negotiations for an agreement were not on track and that he was "quite worried about it".
Mr Rudd has previously warned there were challenges ahead, but publicly was not so pessimistic.
Later he added he had to remain hopeful, but time was running out: "As I said I'm very realistic about it," he said. "The clock is ticking and time is running out."
He urged leaders of all countries to get behind the Copenhagen process warning the consequences of failure would be disastrous.
Leaders at the G8 summit in Italy agreed this week to limit global temperature rises to 2 degrees Celsius above pre-industrial levels.
There is little detail, but it is reported developed countries will aim for the biggest emissions cuts - 80% - while developing nations will be required to reduce their emissions by 50% by 2050.
United Nation Secretary General Ban Ki-moon said on Thursday that progress on climate change at the G8 was "not enough".