LONDON — Britain is experiencing its warmest year in more than three centuries and will likely break a record at the world’s oldest registry of temperatures.

The mean temperature for 2006 will be 51.5 degrees Fahrenheit based on current weather patterns, making it the warmest since records began in the year 1659, preliminary figures released yesterday by Britain’s weather forecaster and the University of East Anglia show.

The phenomenon isn’t limited to Britain. The global average temperature also is heading for one of the warmest years on record, the Met Office, the government’s forecasting arm, said.

“When you see the records that have fallen this year — monthly records, seasonal records — they lead to the inescapable conclusion that temperatures are on their way up,” Barry Grommett, a Met Office meteorologist, said in a phone interview. “The trends that we’ve seen are quite remarkable.”

This July’s mean temperature of 67.5 degrees in Britain made it the warmest month ever, and the 54.7-degree autumn was a record for that season, the statistics show. Warmer weather may curtail demand for natural gas in Britain, the European Union’s biggest consumer of that fuel. Gas is used to heat about three-quarters of British homes and powers almost 40% of its electricity plants.

A record in 2006 would beat the previous two hottest years of 1990 and 1999, which both had means of 51.1 degrees, according to figures from the so-called Central England Temperature statistics.

Worldwide, the 10 warmest years since 1850 all occurred in the past 12 years, the Met Office said. This year, the global average temperature is 32.8 degrees above the 57.2-degree average for the 1961–1990 period, making it the sixthhottest on record, the Met Office said.