A huge drop in the number of butterflies in the UK is causing confusion among wildlife experts, BBC informs.

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.

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.

The worst hit species was the common blue, down by 55%, with five other types dropping by 40% or more.

Mr Fox added: "Butterfly numbers are under pressure from industrialised farming and the concern is that a bad year will do lasting damage."