The British government is being called to show "moral leadership" at crunch climate talks taking place in Glasgow.
Andy Atkins, CEO of A Rocha UK, said a draft agreement published on Wednesday revealed the "yawning gap" between what needs to be agreed at the UN's COP26 summit, and what is currently on offer.
The text, released by COP26 president Alok Sharma, urges countries to strengthen their targets for cutting greenhouse gas emissions by 2030 if global warming is to be kept to 1.5C, as agreed in Paris in 2015.
"The president of COP is absolutely right to send up a distress flare and say that we really are in trouble and we need to do something," said Atkins.
With international emissions targets reviewed only once every five years by the UN, Atkins warned that the world did not have this long to wait.
"It will be too late by then. 1.5C will be dead," he said.
The draft agreement also urges nations to accelerate the phasing out of coal and fossil fuel subsidies, and to increase financial support for countries vulnerable to the harmful effects of climate change.
It follows the as yet unfulfilled pledge made by developed countries in 2009 to provide $100bn a year in climate finance by 2020.
Atkins said that even if the target for financial aid had been reached, the amount would be "nothing like enough" because of the increasing severity of climate change in the last decade.
"Much, much more money is needed," he said.
"But it would also be helpful if the Prime Minister announced a reversal of cuts to British aid.
"It was an extraordinary own-goal and one of the most damaging things Boris Johnson did in the lead-up to COP26.
"If he wants to galvanise things at COP26, he should put it back up immediately."
But Atkins also described a sense of frustration at COP26 over Johnson's "completely inconsistent" stance on climate change, and said that the Prime Minister was potentially "doing damage" at the talks by simply "hectoring" other countries while failing to offer "bold" commitments from Britain.
"It's not ok to say one thing and do the opposite, and it's not ok to come here and tell other countries what to do when Britain itself is way off track in its own short-term target," he said.
"The Prime Minister has committed Britain to net zero by 2050 but the reality is that if we don't cut greenhouse gas emissions very sharply before then, 2050 won't matter, it will already be way too late.
He suggested Johnson "bridge the gap between British rhetoric and British practice" by committing to an end to further fossil fuel exploration and abandoning plans for a new oil field in the North Sea.
"That would galvanise things, because unless Johnson takes a bold, practical step himself, I cannot see how others are going to," Atkins said.
He added, "This is about moral leadership. We need integrity and we need consistency.
"The Prime Minister needs to stop telling other people what to do and start telling people what Britain is going to do."
