Carbon emissions must start to fall within 10 years to limit the global temperature rise to the crucial 2C mark, experts at the Met Office warned.
Even if emissions peaked in 2020, there would be a 50/50 chance of temperatures rising by more than two degrees and heralding potential global catastrophe, scientists told the Copenhagen climate change conference.
Leading industrialised nations have agreed that global warming should be limited to 2C above pre-industrial levels to avoid "dangerous" climate change, and it is one of issues being debated in Denmark.
The Met Office's Avoid study - carried out with several other British scientific groups - found that limiting the rise to 1.5C was now "virtually impossible" because of all the greenhouse gases already pumped out.
That goal has been called for by some poor African countries and small island states - areas of the world seen as being most at risk from global warming.
Even if emissions were suddenly reduced to nothing, there would still be a rise of 1.3C because of the existing greenhouse gases in the atmosphere, Avoid has found.
Researchers calculated that if emissions started falling by 4% per year after 2018 that would give a 50% chance of keeping warming below 2C.
But if the peak came just two years later in 2020, the decline would then have to be 5% per year to deliver the same odds.
Vicky Pope, the Met Office's head of climate science, told the BBC: "If you go to 2025 before peaking, it's virtually impossible to stay under 2C."
She added: "There's no way you'd get a 50 [er cent chance of avoiding 1.5C."
Delegates in Copenhagen heard that ensuring a reasonable chance of keeping the temperature rise under 1.5C would involve "negative emissions" - or removing CO2 from the air.
If carbon output peaks later than 2020, "geo-engineering" techniques such as mirrors in space and artificial trees would be needed to hit the 2C target.
More than 190 countries are gathered in Copenhagen for UN climate change talks.
Earlier this week the Met Office said the past decade has been the warmest period in the 160-year record of global surface temperatures.