The US and Russia have thrown climate talks into disarray by allying with Saudi Arabia and Kuwait to water down approval of a landmark report on the need to keep global warming below 1.5C.

After a heated two-and-a-half-hour debate on Saturday night, the backwards step by the four major oil producers shocked delegates at the UN climate conference in Katowice as ministers flew in for the final week of high-level discussions.

It has also raised fears among scientists that the US president, Donald Trump, is going from passively withdrawing from climate talks to actively undermining them alongside a coalition of climate deniers.

Two months ago, representatives from the world’s governments hugged after agreeing on the 1.5C report by the UN Intergovernmental Panel on Climate Change (IPCC), commissioned to spell out the dire consequences should that level of warming be exceeded and how it can be avoided.

Reaching a global consensus was a painstaking process involving thousands of scientists sifting through years of research and diplomats working through the night to ensure the wording was acceptable to all nations.

But when it was submitted to the UN Framework Convention on Climate Change on Saturday, the four oil allies – with Saudi Arabia as the most obdurate – rejected a motion to “welcome” the study. Instead, they said it should merely be “noted”, which would make it much easier for governments to ignore. The motion has not yet been able to pass as a result of the lack of consensus.

It opened up a rift at the talks that will be hard to close in the coming five days. During the plenary, the EU, a bloc of the 47 least developed countries, as well as African and Latin and South American nations, all spoke in favour of the report. Several denounced the four countries trying to dilute its importance.

Rueanna Haynes, a delegate for St Kitts and Nevis, told the plenary it was “ludicrous” not to welcome a report that UN member nations had commissioned two years earlier and to hold up crucial talks over two words.

“It’s very frustrating that we are not able to take into account the report’s findings: we are talking about the future of the world – it sounds like hyperbole when I say it, but that’s how serious it is,” she told the Guardian. “I would say that this issue has to be resolved. This is going to drag out and the success of the COP is going to hang on this as well as other issues.”

Scientists were also outraged. “It is troubling. Saudi Arabia has always had bad behaviour in climate talks, but it could be overruled when it was alone or just with Kuwait. That it has now been joined by the US and Russia is much more dangerous,” said Alden Meyer, the director of strategy and policy in the Union of Concerned Scientists.

He said the shift in the US position would be embarrassing for the country if it persisted. “Donald Trump is the denier-in-chief. He takes a personal interest in dissing scientists,” said Meyer. “But the science won’t go away. The law of thermodynamics can’t be ignored.”

Climate campaigners said the four blocking governments had been overrun by fossil-fuel interests and were trying to sideline the study.

May Boeve, the executive director of the activist group 350.org, said: “Deliberately ignoring the IPCC report would be wholly irresponsible and 350.org stands with the rest of world in condemning these climate deniers … and the vested fossil fuel interests behind them.”

Ministers have only five days to establish a rulebook for the Paris agreement. A wild card is the role of the host nation, Poland – the most coal-dependant nation in Europe – which will chair the final week of the meeting.

“The big challenge now is for the Polish presidency to set aside its obsession with coal, get out of the way and allow full acknowledgement of the IPCC 1.5C report, and its implications for increasing the ambition of all countries, in the conclusion of COP24 later this week,” said Bill Hare, the managing director of Climate Analytics.

As well as acceptance of the report, there are several other potential fights brewing regarding transparency rules for reporting emissions and proposals for wealthy high emitters to provide financial support to poorer nations struggling to adapt.