Poor countries have accused the rich world of “backsliding” and betrayal of their climate commitments, as they desperately tried to keep alive a long-awaited deal to cut carbon from shipping.

Nations from 175 countries have gathered in London this week at the International Maritime Organisation (IMO) to hammer out the final details of a deal, more than a decade in the making, that could finally deliver a plan to decarbonise shipping over the next 25 years.

If the most ambitious proposals are realised, the agreement would also require all ships to pay a small charge based on the greenhouse gases they emit, with the proceeds going to fund climate action in poor countries. This levy is seen as a crucial source of funding for poor countries, which are seeing increasing economic devastation from extreme weather.

But powerful economies, including China, Brazil and Saudi Arabia, oppose the levy, while others, including the EU, may agree to drastically water it down.

The IMO talks formally open on Monday, and are scheduled to end on Friday. They are the culmination of a series of attempts to rein in carbon from shipping, which have been going on for more than a decade. Shipping accounts for more than 2% of global emissions – roughly the same share as Japan, and set to grow further without urgent action.

Poor countries fear the negotiations are already falling apart, and that they will be the losers. Ambassador Albon Ishoda from the Marshall Islands, speaking for an alliance of Pacific and Caribbean small island states, said rich nations and large developing countries were “backsliding” on previous promises.

“It is difficult to understand what these countries are thinking,” he told the Guardian. “Maybe they are worried about their national sovereignty. But we are basing our argument [for decarbonisation and a levy on shipping] on scientific grounds. The most vulnerable countries are acting as the adults in the room.”

He pointed out that governments in 2023 agreed a roadmap to decarbonise shipping by 2050, and urged them to fulfil it. “We need them [the big economies] to start showing leadership,” he said. “We can all see what is right – let them do what is right.”

Brazil, China, Saudi Arabia and allies have fought the levy on the basis that it could raise prices to consumers. The EU is officially still in favour of a levy, but the Guardian understands it could opt for a compromise that would water down the proposal.

Simon Kofe, minister for transport for Tuvalu, another member of the 6Pac+ alliance of small islands, said a levy on shipping would have minimal effect on the prices of goods to consumers.

He said: “Concerns about the impact of a levy on trade and consumer prices are understandable, but they are also overstated.”

Shipping makes up only about 1% to 5% of the final price of most consumer goods, and the switch to low-carbon technology is forecast by the IMO to raise shipping costs by only between 1% and 9%. At $150 per tonne of carbon, a levy would have only a minimal impact on end prices, according to Kofe: if a $100 pair of shoes, shipped across the world, includes $3 as the shipping cost, the levy would increase that to just $3.72 – so the new retail price of the shoes would be $100.72, he said.

“[Our levy proposal] ensures that the cost of pollution is borne by those responsible. By placing a levy directly on emissions, we uphold the principles of fairness, accountability and climate justice, ensuring no country is left behind in the transition to a cleaner future,” he said.

The IMO talks take place against an increasingly unsettled backdrop for international trade, as US president Donald Trump last week imposed sweeping tariffs on exports to the US from nearly all countries, except Russia and South Korea. The Guardian understands that the US is not playing an obstructive role at the IMO talks.

Arsenio Dominguez, secretary-general of the IMO, said he was confident the meeting would produce the long-awaited settlement on decarbonising shipping. “Just under two years ago, IMO member states were decisive in their commitments in the IMO 2023 greenhouse gas strategy to approve at [this year’s meeting] midterm measures to cut emissions from ships, including a global fuel standard and an emission pricing mechanism.

“These would be the first set of binding measures for an entire global industry to shift to zero or low-carbon fuels and energy sources, on the basis of mandatory emission targets. The measures are more than climate aspirations – they will become mandatory for ships operating globally. [This] week we will take another positive step that will set the course for a net zero future for the maritime sector.”

With the next few days likely to be taken up by highly technical negotiations, however, time is running out. Constance Dijkstra, shipping manager at the Transport and Environment thinktank, said: “We have reached extra time in the IMO negotiations and the IMO is sleepwalking into failure by wasting its last chance to decarbonise the shipping sector. A carbon levy would be a major step in the right direction.”

The talks are scheduled to end on Friday, but whatever the outcome, it will still take many months to implement. Under the complex IMO rules, any deal struck this week will be subject to refinement by officials and member states, until a further meeting this October, where the new settlement can be formally adopted.