Prime Minister Theresa May will push world leaders to make a stronger commitment on tackling climate change.
At the G20 summit in Osaka, she will call on them to set targets for net zero greenhouse gas emissions.
Mrs May has also raised the killing of journalist Jamal Khashoggi with Saudi Arabia.
And she pledged more than £1.4bn to help end global Aids, tuberculosis and malaria epidemics.
The UK has enshrined in law a 2050 target to cut greenhouse gas emissions to zero – or in the most difficult cases, to offset them by planting trees or absorbing CO2 out of the atmosphere.
Mrs May is calling for other world leaders to follow suit and wants the summit’s joint statement to have “the strongest wording we can deliver” on climate change.
But she faces opposition from US President Donald Trump, who is reportedly pushing to water down the commitment from world leaders.
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Ahead of leading a session on climate change at the summit later, Mrs May told her counterparts “we are running out of time to act” on the issue.
“I urge everyone here to push for ambition and consider setting their own net zero targets,” she said.
In a 20-minute meeting with Saudi crown prince Mohammed Bin Salman, she also raised the case of murdered journalist Jamal Khashoggi, a British official said.
US intelligence concluded the crown prince directed the killing of the Washington Post columnist at the Saudi consulate in Turkey last year, but Saudi authorities have denied they were acting on his orders.
With 11 unidentified people put on trial behind closed doors, Mrs May told Prince Mohammed that the legal process must be “open and transparent”, the official said.
In her speech at the summit, Mrs May will also call for other countries to follow the UK’s pledge of £1.4bn to the Global Fund, an international organisation which fights three of the world’s deadliest diseases.
The UK will contribute £467m a year for three years, providing tuberculosis treatment for more than two million people, 90 million mosquito nets to protect people from malaria, and treatment for more than three million people living with HIV – the virus which causes Aids.
Mrs May said the world needed “urgent international action and a truly collective response” to halt the spread of these illnesses.
The pledge follows an appeal by Sir Elton John and French President Emmanuel Macron for an £11bn cash injection for the Global Fund, which is expected to help save 16 million lives.
Sir Elton, whose Aids foundation works with the fund, said the UK’s response set an “extraordinary example for others to follow”.
UK presses world leaders for stronger climate change action}