Scientists have previously said climate change had caused a widespread loss of bumblebees across the world.
The insects, an important pollinator, normally hibernate through winter into spring.
The Christmas period was unseasonably mild, but since then there have been days of cold and snowy weather.
Buglife said at least two of the UK’s 25 species of bumblebee were believed to have started nest-building early.
The charity’s Paul Hetherington said: “In 2019 Buglife ran a bee survey over the Christmas to New Year break and were amazed at the results with honeybees and bumblebees found from Jersey through to Thurso.
“This year one of my colleagues based in Aberdeen spotted active worker bumblebees over the Christmas break.
“The fact that there are active workers means not only have the queens awakened from hibernation but they have gone to the extent of starting new nests.”
Mr Hetherington said a lack of flowers and wintry weather left the nests at risk of collapse and could kill the bees.
He added: “If this happens there will be no new queens produced meaning far fewer to emerge in spring further feeding the known decline in bumblebees.”
