Scientists and NGOs have accused the UN’s environment programme (Unep) of locking out those “most needing to be heard” from upcoming negotiations in Paris aimed at halting plastic waste.

Last-minute restrictions to the numbers of NGOs attending what the head of Unep described as the “most important multilateral environmental deal” in a decade will exclude people from communities in developing countries harmed by dumping and burning of plastic waste as well as marginalised waste pickers, who are crucial to recycling, from fully participating, they said.

The groups criticised the agency for publishing a report this week, before negotiations between 193 countries over 29 May to 2 June, which they claimed did not fully reflect the health and environmental effects of plastic pollution. The report said mismanaged plastic waste could be slashed by 80% by 2040.

Scientists’s Coalition for an Effective Plastics Treaty (Scept), representing 200 scientists who were invited to comment before the report’s publication, said their concerns and criticisms were ignored.

Unep said it regretted that “due to a technical issue” an email containing Scept comments was not received in time for publication. However, it said it received feedback from 75 experts from 39 organisations that were incorporated. It denied claims its report did not sufficiently reflect the health and environmental impacts of plastic.

Rich Gower, a senior economist for Tearfund, an international NGO that provides advocacy and support to waste pickers, said: “This last-minute restriction locks out those who most need to be heard. It’s vital that negotiators hear from those with firsthand experience of plastic pollution: waste pickers, communities harmed by dumping and burning, and those living near toxic production plants.

“Without these groups’ voices the treaty will fail to be the life-changing instrument we desperately need it to be.”

The Tearfund partner and campaigner Tiwonge Mzumara-Gawa, from Malawi, who will now share a pass with four colleagues after NGOs were told only one in five people would have access to negotiations, said: “World leaders must hear real stories from people in countries where the plastics treaty could have a life-changing impact. In Malawi we don’t have organised public waste collection and many people are forced to either dump or burn their waste. It’s extremely disappointing that my colleagues and I will now have much less access to these talks.”

Therese Karlsson, a science adviser at International Pollutants Elimination Network and member of Scept, said: “We were asked to review the report. Thirty scientists provided over 300 comments. We have several major concerns, one around the framing of the report, which is very focused on technological solutions in a very optimistic way, even although those technical solutions are not proven.”

“This creates a narrative that makes is easy to think we can fix our way out of this. But we need to talk about chemicals from plastics and decrease the use of plastics.”

One concern, she said, was the inclusion of chemical recycling, which was “not recognised as environmentally sound management” under the Basel convention, as a possible interim solution.

Bethanie Carney Almroth, a professor of toxicology at the University of Gothenburg in Sweden and a member of Scept, said: “For a group that have called on scientists, they have locked us out of the room and they have not registered our criticisms.”.

Andrés Del Castillo, a senior attorney at the Center for International Environmental Law, said: “The headline figure of the Unep report predicts an 80% reduction of plastics pollution with the proposed ‘new plastics economy’. This number does not represent a reduction, but rather a narrowing of the scope and definition of what is considered plastic pollution from a full life cycle perspective.”

The “reduction”, which refers to a decrease in plastic waste into the environment, did not count other elements of plastic pollution, such as upstream greenhouse gas emissions and toxic emissions, he said.

NGOs have expressed concern over the report’s inclusion of burning plastic waste in cement kilns as one of several strategies to address the plastics crisis.

Dr Neil Tangri, the science and policy director at Global Alliance for Incinerator Alternatives, said: “Burning plastic waste in cement kilns is a ‘get out of jail free card’ for the plastic industry to keep ramping up plastic production by claiming that the plastic problem can be simply burned away.”

A spokesperson for Unep said NGOs were informed at registration that the venue, which has a capacity of 1,500, had space constraints. However, 640 or 40% of attendees were from NGOs, it said. It denied claims its report insufficiently considered the health and environmental impacts of plastic pollution.

It said: “A large part of the economic costs associated with plastics pollution, assessed at the outset of the report, are due to human health impacts … well over one-third of the estimated $300bn a year costs stemming from plastic pollution are related to exposure to hazardous chemicals.

“The report builds on this evidence to call for the need to completely transform the plastics economy. On waste management the report proposes changes that affect the whole life cycle, from raw materials extraction and processing, with a significant focus on design needs (design of both products and systems), use and reuse, and indeed highlighting the importance of strengthening waste collection and management.”

Unep denied the report presented chemical recycling and cement kilns burning plastic as fuel as part of the solution to plastics problem, saying they were simply “options when safe disposal facilities are to be used as a last resort to avoid plastic leaking into the environment and stresses the needs for emissions (and/or effluent) control and standards to assure safety.

“The potential lock-in effect of purpose-built facilities such as incinerators and plastic-to-fuel plants is highlighted as something to be avoided,” it said.