Foreign Secretary William Hague said the IPPC's report "confirms that climate change is already happening, as a result of human activity".
"The IPCC's report makes clear that unless we act now to reduce carbon emissions, all this will continue to worsen in coming decades," he added.
US Secretary of State John Kerry called the findings "a wake-up call".
"Those who deny the science or choose excuses over action are playing with fire," Mr Kerry said, referring to sceptics who question the need for urgent action on climate change.
Presenting the most vivid evidence to date, the UN's IPCC report has warned that humans are responsible for changes to the earth's climate.
The UN IPPC report into climate change, due out tomorrow, will say scientists agree the world is getting warmer, and humans are to blame.