Canada is worried about an unconditional declaration that access to safe drinking water is a human right. The Holy See is against using family planning to advance gender equality. And dozens of countries are wary about getting rid of fossil fuel subsidies.

These are just some of the objections negotiators have raised ahead of this week’s Rio+20 sustainable development conference, which the UN says is the biggest event it has ever organised.

They underline the growing doubts many have about what the 100-plus leaders expected to fly in for the meeting will have achieved by the time it ends on Friday. “It’s like a rally race of back seat drivers,” said Lasse Gustavsson, head of the World Wildlife Fund International delegation. “Everyone is sitting in the back seat and no one is taking responsibility.”

The summit gets its name from being held 20 years after the 1992 Rio earth summit that launched a number of landmark treaties, including ones to limit the extinction of species and climate change.

But progress has been so slow that only four of the world’s 90 most important green goals and objectives have seen significant progress, a UN Environment Programme report said earlier this month and there has been “little or no” improvement on goals to address 24 problems including severely depleted fish stocks, climate change and deteriorating coral reefs.

Yet despite months of negotiations leading up to this summit, officials have struggled even to finalise the wording of a far less ambitious final text document on the eve of the formal conference opening on Wednesday.

At other global green summits, the US might have been blamed. But this year, Canada is under fire for what environmental campaigners like to call a “blocking mentality”.

A commitment recognising the human right to safe drinking water has survived to the latest draft text, but only with the added condition that it does not relate to “transboundary water issues”.

Water-rich Canada insisted on this, say people close to the negotiations, adding it appeared Ottawa was concerned about potential legal problems surrounding any effort to export water abroad.

A spokesman for Canada’s environment ministry told the Financial Times that Canada supported the human right of everyone to safe drinking water.

But he added: “We recognise that the right to safe drinking water and basic sanitation does not encompass transboundary water issues including bulk water trade, nor any mandatory allocation of international development assistance.”

The Vatican’s efforts to influence negotiations at a summit trying to address the impact of forecast population rises on pressed natural resources and poverty has also proved contentious in some quarters.

“I am baffled that the Holy See is taken seriously and allowed real influence in this field,” said Roger Martin, a former British diplomat who chairs the Population Matters campaign group. “That a body representing a group of old, celibate men should set themselves up as a world authority on all matters sexual is surely ludicrous.”

One of the most hotly contested sections of the summit text has been the paragraph on phasing out environmentally harmful fossil fuels – a move the G20 backed three years ago that has proved difficult to implement.

The latest draft Rio text says the summit should “recognise the need for further action” on such subsidies, “taking fully into account the specific conditions and different levels of development of individual countries”, but adds: “Note: placement of paragraph still to be determined”.

The EU, one of the pushiest advocates at global green summits, has meanwhile been distracted by the Greek election’s impact on the eurozone crisis, prompting something of a parlour game about which of its leaders will make it to Rio+20.

France’s newly elected president François Hollande was still expected, as were the leaders of Sweden and Denmark, as well as European Commission president José Manuel Barroso.

But Germany’s chancellor Angela Merkel and the UK’s prime minister David Cameron are among those who have decided that, like US president Barack Obama, they are better off staying at home.