10 October 2016
The Big Butterfly Count - an annual survey by thousands of volunteers - recorded an average 12.2 per count, compared with a 2013 high of 23.
Numbers were even lower than a previous slump in the wet summer of 2012, despite far warmer weather.
Butterfly Conservation, which organised the count, said the cause was still a mystery.
Image copyright Butterfly Conservation Image caption
The count involved more than 36,000 people who recorded more than 390,000 butterflies.
The survey has been running since 2010 but the group uses nationwide figures which go back to the 1970s.
Butterfly Conservation's head of recording Richard Fox said: "The overall trend has been a decline but we expect annual variations based on the weather.
"But the fall this year has been shocking and disappointing as the summer weather has been good, and we don't know why.
"The most extreme thing was the exceptionally mild winter but it is debated if that is a good or bad thing for butterflies."