Sightings of the reptiles at Humberhead Peatlands, near Thorne in South Yorkshire, and Skipwith Common, a new nature reserve outside York, have increased fourfold.
Steve Hiner, a reptile expert from Natural England, said: “Adders and common lizards are really quite rare and shy so it’s always a joy and privilege to see them when they emerge to bask and glint in the sunshine. We’re pleased that they are responding to the work we’ve been doing to improve their habitat.”
He added: “There have been some good sightings of them this year – we’ve had some good days of sunshine and they’re reluctant to go into hibernation just yet.”
Natural England is also asking people not to get too close to any adders, if they were to see the reptiles.
Mr Hiner said: “Adders are Britain’s only venomous snake and could attack if they feel threatened. They can be admired from a safe distance.”