The UK has ratified the world's first comprehensive treaty on tackling climate change, the Government said.
Foreign Secretary Boris Johnson has signed the Paris Agreement, which commits countries to limiting global temperature rises to "well below" 2C over pre-industrial levels and pursue efforts to keep increases to 1.5C.
More than 100 countries have already ratified the deal, which came into force earlier this month, and the move by the UK was welcomed by campaigners amid concerns the election of Donald Trump as US president could put international climate action at risk.
The UK's ratification document is now being sent to the United Nations in New York.
It comes as countries meet in Morocco for the latest round of UN climate negotiations, focused on implementing the Paris Agreement.
Climate Change and Industry Minister Nick Hurd, who is at the talks in Marrakesh, said: "The UK is ratifying the historic Paris Agreement so that we can help to accelerate global action on climate change and deliver on our commitments to create a safer, more prosperous future for us all."
He said the talks were an " important milestone which marks the shift from aspiration to implementation" on driving down emissions.
"We are going to use this positive momentum to grow the UK low-carbon sector, which is already worth over £46 billion, as we continue to provide secure, affordable and clean energy to our families and businesses."
In order to meet the Paris Agreement's temperature limits, the world will have to reduce greenhouse gas emissions to net zero in the second half of the century, which will involve huge transformations in global electricity, transport, heating, agriculture and industry.
The move by the UK comes against a backdrop of political upheaval in the US, which had seen President Barack Obama push forward on global action to drive down greenhouse gases in close partnership with the Chinese.
President-elect Donald Trump has previously claimed global warming was a hoax made up by the Chinese to make US manufacturing uncompetitive, and has promised to boost polluting coal and pull out of the Paris Agreement.
But other countries, leading businesses and US Secretary of State John Kerry have all warned in the past few days of the need for urgent action to support the shift to a low-carbon economy and avoid the dangerous impacts of climate change.
Business representatives and environmental and anti-poverty campaigners welcomed the move by the UK.
Dr Stephen Cornelius, chief adviser on climate at WWF-UK, said: "It has never been more important for the world to stand together on climate change, and the UK joining the Paris Agreement is a welcome signal.
"As the first industrialised nation to commit to phase out coal, and a key architect of Paris, the UK is well placed to show courage and leadership by fronting a diplomatic push for low-carbon policies worldwide.
"This must be backed by urgent action to decarbonise our domestic economy and to help firms capitalise on huge and growing markets in low-carbon technology.
Jill Duggan, director of The Prince of Wales's Corporate Leaders Group, with 24 global businesses employing two million people worldwide, said: "The UK Government's ratification of the agreement today sends an important signal to international allies, businesses and investors about the inevitable transition towards a zero-carbon economy."
Mohamed Adow, Christian Aid's international climate lead, said the move by the UK - "America's oldest and strongest ally" - showed global support for action on climate change remained resolute.
He said a number of countries had ratified the Paris Agreement since the US election, including oil-rich Saudi Arabia.
"If a modern, post-Brexit Britain is to thrive, it needs to be at the forefront of the global clean energy transition, something the UK's low-carbon sector is well placed to take advantage of.
"Market forces are clearly pointing towards the smart money being in clean tech.
"The country that dominated the first industrial revolution is now positioning itself to capitalise on the second," he said.