Great Britain's largest and rarest butterfly is facing a new threat as its source of food is being attacked.

Swallowtail, which is found only in the Norfolk Broads, is at further risk of extinction following a fungal pathogens outbreak which has drastically caused its foodplant, milk parsley, to die-back.

The species which is classified as "vulnerable" to extinction in Britain could be placed at increased risk because of the new disease.

Milk parsley is a protected plant that grows in the Broads and is the only plant that swallowtail caterpillars feed on. If it vanishes the rare species risks becoming extinct.

Eastern Daily Press: A swallowtail on milk parsley, a plant it is entirely dependent onA swallowtail on milk parsley, a plant it is entirely dependent on (Image: Archant)

The 'droop' is believed to be caused by a known fungal pathogen joining forces with another pathogen, or saltwater 'inundations' from flooding.

It has drastically knocked back milk parsley at Wheatfen Nature Reserve, near Norwich.

Will Fitch, warden at Wheatfen, told the Guardian that he is closely monitoring the site for further outbreaks.

He said while the plant is not thriving, the site had a "good population of swallowtails this year and caterpillars on the plants" and hopes the problem is "not the end of the world".

It comes after the latest Red List for British butterflies shows a worsening picture for many species and climate change and nitrogen pollution adding to the pressure for insects.

Andy Brazil, county recorder at Norfolk's Butterfly Conservation branch, said the Butterfly Conservation (BC) is aware of the issue at Wheatfen.

Eastern Daily Press: Andy Brazil, Norfolk's county butterfly recorder, searches for butterfliesAndy Brazil, Norfolk's county butterfly recorder, searches for butterflies (Image: Archant)

He said: "At present it seems confined to just that one site, but we won't know until later in the year.

"The Norfolk swallowtail is entirely dependent on the plant, so their fortunes are intimately bound together - if the milk parsley goes, so will the butterfly."

The county recorder said the fungus has "struck" before, when there was an outbreak in the 1930s, and said the swallowtail population had recovered.

"So although the situation is worrying, it's not yet dire," Mr Brazil said.

"Of course, back then there were a lot more swallowtails - since dedicated monitoring began in the 1970s, BC has recorded a fall of over half in the total swallowtail population.

Eastern Daily Press: Swallowtail butterfly feeding on the Norfolk Broads near Horning. Photo by Bill SmithSwallowtail butterfly feeding on the Norfolk Broads near Horning. Photo by Bill Smith (Image: Archant © 2011)

"But in the last few years numbers appear to have stabilised, and all the breeding sites are now in conservation management."

He added that research is currently under way at the University of East Anglia with the aim of finding ways to encourage milk parsley, which is supported by Norfolk BC and the Swallowtail and Birdwing Trust.

Kew and the Royal Horticultural Society are also studying the disease.