The striking orange and black wings of the high brown fritillary were once a familiar sight in woodlands across England and Wales.
But since the 1950s the butterfly’s numbers have fallen dramatically and it is now only found on around 30 sites in the UK, leading conservationists to fear it could become extinct.
A new study has revealed that 2014 was the best year for a decade for the species, with its population soaring by more than 180% compared to the previous year.
The high brown fritillary (Fabriciana adippe) is one of only two critically endangered butterflies in the UK, and is restricted to colonies in the north west and south west of England, and one in Wales.
It mainly breeds on moorland slopes where violets grow underneath bracken, or in short vegetation in woodland clearings.
The UK Butterfly Monitoring Scheme’s (UKBMS) findings have been put down to last year’s warm spring weather and work undertaken to restore its habitat.
Damp weather in May could also have helped as research has shown there is a positive association between the butterfly's abundance and wetter weather during the spring and early summer period.
“There is a long way to go before the long-term decline has been reversed, with ongoing targeted conservation efforts crucial in this,” said Dr Tom Brereton from Butterfly Conservation.
The UKBMS started in 1976 and uses data collected from around 2,250 UK sites throughout the summer.
More than half of the 56 species it studied last year saw their numbers rise, compared to 2013.
A warm start to the summer saw the butterfly season peak in July rather than August, which was colder and wetter than average.
The marbled white (Melanargia galathea), ringlet (Aphantopus hyperantus) and brimstone
(Gonepteryx rhamni) butterflies all experienced their best years since the monitoring scheme started.
Orange-tip (Anthocharis cardamines) and speckled wood (Pararge aegeria) butterflies had their fourth-best years on record, and the threatened Duke of Burgundy (Hamearis lucina) rallied with a 26% increase in numbers.
All seven species of skipper butterflies that were studied showed an annual increase, the large skipper (Ochlodes sylvanus) population soared by 86% and numbers of the Lulworth skipper (Thymelicus acteon) climbed by 15%.
The butterflies that lost out last year included the 'cabbage white' butterflies, with numbers of the small white (Pieris rapae) down 66% and the large white (Pieris brassicae) declining by 69%.
Cold temperatures in August also saw some high-summer butterflies struggle, with the chalk hill blue (Polyommatus coridon) population falling by 55% and Adonis blue (Polyommatus bellargus) numbers dropping 43%.
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