Hundreds of birds are dying from a parasite that leaves species lethargic and kills them within three weeks.
Vets are now investigating the deaths of the garden birds
Birdwatchers and residents have reported dead sparrows, chaffinches and goldfinches, mainly in south west England and the Midlands.
The parasite, known as trichomoniasis, is not linked to avian flu and poses no risk to humans, experts say.
The Institute of Zoology has begun an investigation into the deaths to try to understand why the disease has emerged.
A record finch death rate was identified by the Garden Bird Health initiative - a three year project co-ordinated by the Institute of Zoology, which is part of the Zoological Society of London.
Becki Lawson, a wildlife veterinarian from the institute, said: "The dramatic increase in the number of finch mortality incidents reported by members of the public is deeply concerning.
"It is essential that further research is now undertaken so that we can gain a better understanding of the reasons for the disease emergence, and to assess the parasite's impact on our bird populations."
The Royal Society for the Protection of Birds (RSPB) has also emphasised that bird baths and feeders should be regularly washed and moved around the garden to guard against infection.
'Unusually hot weather'
Andre Farrar, spokesman for the RSPB, said: "It is an unusually large outbreak.
"Trichomoniasis usually affects pigeons and doves, but this summer it seems to have affected finches and house sparrows.
"There is no human health risk but, as always, when handling birds people must take care."
Mr Farrar added the unusually hot weather in Britain this year could have contributed to the spread of the disease.
A large number of cases have also been reported in Wales.