Theresa May said she was dismayed about Donald Trump's decision to pull out of an international agreement to tackle climate change as the US president was left isolated on the issue at the G20 summit.
The Prime Minister said she urged Mr Trump to rejoin the Paris climate change agreement and raised it with him on the sidelines of the gathering in Hamburg, although it was not one of the items on the agenda in the formal meeting between the two leaders.
She said: "Like other world leaders here, I am dismayed at the US decision to pull out of the Paris agreement and I have urged President Trump to rejoin the Paris agreement.
"The UK's own commitment to the Paris agreement and tackling global climate change is as strong as ever."
Challenged on why she had not used her bilateral meeting with the US president to tackle him on the issue, she said: "I did bring the issue of the climate change agreement up with President Trump, I've had a number of conversations with him over the time I have been here at the G20.
"When I brought it up with him, what I did was I encouraged him to bring the United States back into the Paris agreement.
"I continue to hope that is exactly what the United States will do."
Although Mrs May did not raise the president's decision to pull out of the Paris Agreement during their 50-minute bilateral meeting, officials said she brought it up as they walked between engagements at the Hamburg summit.
In the summit's official declaration, the leaders of the world's most powerful economies "take note" of the decision of the US to withdraw from the deal, adding that "the leaders of the other G20 members state that the Paris Agreement is irreversible".
German chancellor Angela Merkel said the US president's decision was regrettable, while French president Emmanuel Macron said he would host a summit in December to discuss the next steps on the agenda.
At a closing G20 press conference, Mrs May underlined the UK's commitment to the climate change deal.
"Not only will this protect the environment for future generations, it will keep energy affordable and maintain a secure and reliable supply in order to protect the interests of businesses and consumers," she said.
"We play a leading role internationally and we are delivering on our commitments to create a safer, more prosperous future for us all."
Critics of the US policy highlighted the splits within the international community, saying there was now a G19 committed to the Paris deal with Mr Trump on the outside.
But conservation groups demanded faster action to end the use of fossil fuels.
Greenpeace International executive director Jennifer Morgan, said: "The G19 held the line, defending the Paris Agreement against Trump's backward decision to withdraw, but that is not enough.
"The G19 should have committed to accelerate the transformation away from coal, oil and gas. If Paris was the starting point, Hamburg must sow the seeds of much greater ambition.
"Millions of people suffering from the impacts of climate change are demanding urgent action to end the age of coal, oil and gas. To put words into action, the G19 must now accelerate the clean energy transition and set sail from Hamburg with an agenda of change."
WWF climate and energy spokesman Manuel Pulgar-Vidal said: "Implementing the Paris Agreement is in the interest of each nation.
"Effective climate strategies can help unlock new business and employment opportunities, renewable energy, health benefits, and a sustainable future for all.
"As G20 leaders join cities, companies and individuals around the globe in committing toward a climate-safe future, it must be crystal clear that there is no place for fossil fuels in this scenario. We can be stronger together for climate but we need to translate ambition into action now."
Mrs May was involved in the sensitive discussions to reach agreement on part of the leaders' declaration relating to climate change.
The Prime Minister, along with French president Emmanuel Macron and Australian prime minister Malcolm Turnbull, was involved in talks with Mr Trump to agree the wording of the summit communique on the issue.
A diplomatic source said: "There was concern that not being able to agree the language on the Paris agreement could hold up the whole summit communique.
"Macron asked PM May to step outside with him, Trump and Turnbull to agree a form of words which allowed the 19 to express their support for Paris in the strongest possible terms, with the US stating its own position. This was done."