The Rio+20 summit in Brazil from 20-22 June is expected to draw more than 50,000 participants from governments, companies and environmental and lobby groups.
It will try to hammer out sustainable development goals across seven core themes including food security, water and energy but is not expected to produce mandatory targets.
Informal talks are taking place in New York until 4 May to shape the main negotiating text.
One area of discussion is how to measure economic growth to take into account the value of natural assets, such as water and forests, as well as innovative financing, phasing out fossil fuel subsidies and a plan to help prevent ocean acidification.
Participants said there had been moves to water down a clause which would require companies to include sustainability information in their corporate reporting.
"It appears to be batting to and fro in a tennis rally here," David Pitt-Watson, chairman of investor Hermes Focus Asset Management, told Reuters from New York yesterday (1 May).
"There have been suggestions for watering down and also for putting the clause back in as it originally was," he said.
Investment and pension funds would like the measure agreed so they know as much as possible about the "green" credentials of a company before investing in it, which could help open up their trillion-dollar assets for more clean energy investment.
Steve Waygood, chief responsible investment officer at Aviva Investors, representing a coalition of investors worth $2 trillion which backs the proposal, said: "Currently 75% of companies do not report on sustainability issues at all.
"Without this convention, at the current rate it will be decades before sustainability reporting is common practice across global markets."
The UN Earth Summit in Rio 20 years ago ultimately led to the Kyoto Protocol on capping emissions of planet-warming greenhouse gases and a treaty on biodiversity.
Since then, successive attempts to secure a new binding pact to cut emissions have failed, public interest in climate change has waned and many world leaders are focussed on financial woes.
The Danish EU presidency recognised last month that ambitions amongst EU countries might be waning to define concrete action on water, marine environment, land-use and biodiversity, sustainable energy, resource efficiency and waste management.
Some have grown tired of UN climate meetings after a much-hyped conference in Copenhagen in 2009 failed to deliver a binding deal to halt global warming. Last year, a UN summit in South Africa agreed to forge a new deal by 2015, but it will not enter into force until 2020.
'More ambitious'
The original 19-page draft text has grown to just under 200 pages along with supplementary texts and could be revised at a final round of informal talks in Rio from 13-15 June.
"With only 10 working days left for discussions before the text is presented to Rio, leaders need to be more ambitious about what they hope to achieve," said Lasse Gustavsson, executive director of conservation at WWF International.
Language on phasing out "market distorting and environmentally harmful" subsidies, including for fossil fuels, agriculture and fisheries, could also be weakened or dropped after opposition from Japan, the United States and Canada, observers said.
More than 130 heads of government and deputies are on the summit's speaker list but some major figures, such as US President Barack Obama and British Prime Minister David Cameron, are not expected to attend.
