Global average temperatures in 2017 are expected to be about 0.75C above the long-term average of 14C seen between 1961 and 1990, according to analysis using the new Met Office supercomputer.
It would put it among the warmest years on record, but would not outstrip 2016, when global temperatures were 0.86C above the long-term average between January and October, the experts said.
That is because part of the record warmth in 2016 was caused by a strong El Nino climate phenomenon in the Pacific Ocean which affects the world's weather and drives up global temperatures, but which has now faded.
It is also unlikely to be hotter than 2015, which was also affected by the El Nino, and was the hottest year on record until it was overtaken this year.
Professor Adam Scaife, head of long range performance at the Met Office, said: "This forecast, which uses the new Met Office supercomputer, adds weight our earlier prediction that 2017 will be very warm globally but is unlikely to exceed 2015 and 2016: the two warmest years on record."
Last year, the Met Office predicted 2016 would be about 0.84C hotter than over the long-term, with a range of 0.72C and 0.96C above average.
This year it has predicted 2017 will be between 0.63C and 0.87C above average, with a central estimate of 0.75C.
Professor Chris Folland, Met Office research fellow, said 2016 was well forecast because the methods used to produce the outlook for the year detected the influence of the strong El Nino.
He added: "However, last year's El Nino only accounts for around 0.2C of the global mean temperature rise for 2016, when compared with the long-term average between 1961 and 1990.
"Increasing greenhouse gases are the main cause of warming since pre-industrial times."
The World Meteorological Organisation announced last month that 2016 was set to be the hottest year since records began in the 19th century, as a result of man-made warming and the strong El Nino.
Some 16 of the 17 warmest years on record have occurred in the 21st century, and if 2017 matches the Met Office prediction it will be the third hottest year on record.