Theresa May is "complicit" in Donald Trump's likely withdrawal from the Paris Agreement on climate change by not openly criticising his move, a campaigner has said as protesters gathered outside the US Embassy in London.

President Trump has said he will announce on Thursday whether he intends to go ahead with his election campaign pledge to withdraw from the landmark 2015 deal to reduce CO2 emissions.

The move has sparked alarm around the world, and the Prime Minister reaffirmed Britain's commitment to the deal, which she said she had raised with Mr Trump at a meeting of G7 leaders in Italy last week, adding "it's up to the president of the United States to decide what position the United States is going to take on this matter".

Campaigner Samir Dathi, coordinator of the Stop Trump coalition and one of the demonstrators outside the US Embassy, said other world leaders have said Mr Trump should not pull out of Paris, adding: "She could at least say that."

Mr Dathi told the Press Association: "I think Theresa May is herself someone who doesn't really believe in climate change, doesn't have a sound commitment to it, and is much more interested in profits, in the unfettered ability of companies to make huge amounts of profit."

He added: "I think Theresa May is complicit in what Trump's doing by not firmly criticising his move ... She didn't censure him at all. She didn't criticise him."

Mr Dathi said her reaction was "pathetic", adding that he believed there were "strong elements of climate denial" within her cabinet.

The couple of dozen demonstrators at the US Embassy held banners saying "Trump: Climate Disaster" and "Trump Special Relationship? Just say No".

The White House has signalled Mr Trump is likely to pull out of the agreement, although aides made clear that no decision was final until the president made his announcement.

Mr Dathi said it would be "disastrous" and a "serious blow" to the fight against climate change if Mr Trump pulled the US out of the agreement.

"At least previous leaders, even though their actions have been woefully inadequate, at least they recognised that there was a problem with climate, and something needed to be done," he said, describing Mr Trump as a "complete maverick" on the issue.

Looking ahead to what the US pulling out could mean for the world, Mr Dathi said it would be "apocalypse", adding: "The worst consequence is the destruction of our climate, which is something we've been heading for for a long time."

He said Mr Trump and his team were "a gang of climate deniers" who represented a "uniquely existential threat to life around the world".

He said he believed there was still time to make a difference, but felt powerful social movements would be crucial.

"You can only re-build society and make it more sustainable if there are massive social movements organising from below," he said.

Pressed on the issue during a campaign event in Teesside, Mrs May said: "We are a leading nation in the world in terms of dealing with climate change and it was only at the G7 last week that I was making clear to President Trump, as were other leaders that were sitting around that table, the importance we attach to the Paris Agreement.

"Now it's up to the president of the United States to decide what position the United States is going to take on this matter but we retain our commitment to the Paris Agreement."

Claire James, from the Campaign Against Climate Change, described the potential move as "catastrophic", adding: "We are really on a knife edge here and running out of time to avoid catastrophic climate change."

Ms James said she was "disappointed", but not surprised, that there was not a stronger response from Mrs May regarding Mr Trump's likely withdrawal from the agreement.

"It just comes across as very weak and not showing that international commitment to act on climate change we'd like to see," she said.

Ms James added: "Post-Brexit we know that the UK is going to be looking for new trade deals, and so the relationship with the US is going to be slightly different.

"But it's really, really sad if the UK is not able to stand up and say 'We know the science, it's been clear for decades and we need to act' as many other world leaders are doing."