Its "Corruption Perceptions Index" for 2024 showed that many countries involved in climate action -- either as hosts of the United Nations climate summits or because they are most at risk from rising temperatures -- scored poorly or worse than before.

"Corrupt forces not only shape but often dictate policies and dismantle checks and balances," Transparency International CEO Maira Martini said in a statement.

"We must urgently root out corruption before it fully derails meaningful climate action."

Graft could see funds intended to tackle global warming diverted to other ends and weaken the implementation of environmental regulations, the group said.

The annual report assigns countries with a greater perceived risk of public sector corruption a lower score on a scale from zero to 100.

Brazil, for example, the host of this year's UN COP 30 climate talks, received a score of 34, its lowest-ever rating.

The convener of the previous summit for international climate action, oil-producing Azerbaijan, scored just 22.

Meanwhile, the United Arab Emirates, which appointed the head of the state oil firm to chair the COP it hosted in 2023, had a relatively better score of 68.

The report also highlighted declining scores in wealthier countries which have often taken a leading role in talks, including Canada (75), New Zealand (83) and the United States (65).

Corruption in these countries "undermines global progress the most", the report said.

"These nations hold the greatest responsibility to lead on ambitious climate goals, reduce emissions at scale and build resilience worldwide," it said.

Among the countries most exposed to climate change were also some of those with the worst scores on the index, such as South Sudan (eight), Somalia (nine) and Venezuela (10).

"Marginalised people usually have fewer options for adapting to extreme weather and pollution, leaving them most in need of government support," the report said.

One solution would be to have better "metrics and frameworks for climate transparency and accountability" to better track progress and make sure funds are used properly, the report said.