Donald Trump is pulling the US out of the world's first comprehensive deal on climate change.
President Trump announced the US would withdraw from the Paris Agreement, secured in the French capital in December 2015, which commits countries to curbing rising global temperatures.
But the president, who has previously labelled climate change a hoax by the Chinese to hurt US manufacturing, also raised the possibility of negotiating to re-enter the Paris accord or an entirely new deal on terms that were "fair" to the US.
World leaders criticised the decision, with Prime Minister Theresa May telling Mr Trump in a phone call of her "disappointment" at his decision to pull America out of the Paris Agreement.
In a statement Downing Street said Mrs May had stressed to Mr Trump that the UK remained committed to the deal which "provides the right global framework for protecting the prosperity and security of future generations, while keeping energy affordable and secure for our citizens and businesses".
The president was accused by climate experts of ignoring the science, and of failing to see the opportunities of new clean technologies and of the jobs they provide.
His decision, which would put the US alongside Nicaragua and Syria as the only countries not signed up to the deal, was labelled by environmental campaigners as an "act of vandalism" which put people's lives and livelihoods at risk.
At a press briefing at the White House, Mr Trump said the US would end the implementation of its voluntary commitments under the Paris Agreement to cut greenhouse gas emissions by 26% to 28% by 2025 compared with 2005 levels, and its contribution to climate finance.
He said the deal allowed countries such as China and India to carry on polluting while the US economy was harmed, saying it was "less about the climate and more about other countries obtaining a financial advantage over the US".
The accord commits countries to holding global temperature rises to "well below" 2C above pre-industrial levels, which will require emissions to be cut to net zero by the second half of the century.
Scientists have warned failure to curb dangerous climate change will lead to sea level rises, more intense storms and flooding, more extreme droughts, water shortages and heatwaves as well as massive loss of wildlife and reduction in crop yields, potentially sparking conflict and mass migration.
Despite the decision by the US, the second biggest polluter after China, to pull out of the deal, many analysts suggest the shift to a low carbon economy is now unstoppable, with renewable prices tumbling and new clean technology being developed and deployed.
And former UN climate chief Christiana Figueres, who helped preside over the talks to secure the agreement, said: "The real economy both in the US and internationally must and will continue its decarbonisation trend, pulled much more strongly by market forces than held back by politics.
"States, cities, corporations, investors have been moving in this direction for several years and the dropping prices of renewables versus the high cost of health impacts from fossils guarantees the continuation of the transition," she said.
The EU and China are expected to affirm deeper commitment to climate action at a summit on Friday.