October may have been warm, but it was also wet
The average temperature for the month - 13.2C (55.8F) - has been the highest since records began in 1659, beating the previous best set in 1969.
Rainfall has been high too, but not on the same scale as last October.
Met Office spokesman Andy Yeatman said: "We still have one final day to go but the figures are very likely to confirm a record October."
But the month of November is expected to bring changes, with colder weather on the cards and the first ground frosts for some parts of the UK.
Sunshine
In 1969, when the previous record was set, the average temperature was 13C (55.4F).
This October's average is 2.6C above the normal average for the month, after a series of sunny days.
But October has not been without its showers.
The England and Wales Rain Series - which measures rainfall - puts this month's rainfall 60% above the normal rainfall.
Mr Yeatman said 138mm of rain fell during the month.
"This was high but not as much as last October, when 188mm of rain fell in the whole of the month," he said.
Mild air
"In autumn and winter, warm weather often goes hand in hand with rainfall," he added.
The PA Weather Centre confirmed on Wednesday afternoon that this month had been the warmest ever.
The Met office is expected to confirm its statistics on Thursday morning.
The unusual warmth has been caused by winds blowing from the south or south west for much of the month, bringing very mild air from Spain and the Azores.
Britons have enjoyed temperatures of 13.6C in central England, with Herne Bay in Kent achieving the highest temperature of 25.3C on 13 October.
Mr Yeatman said: "A temperature like this in October is a very rare event. It is a record, but it follows a trend in the last 100 years for quite warm Octobers. Last October was the exception.
"This weather extreme is another piece of the jigsaw towards the global warming theory."