The egret chick seen at Shapwick Heath National Nature Reserve is the first of its kind to hatch in the country, they said.
There have been unconfirmed sightings of a second chick in the nest, they said.
"It was a great sense of relief when we confirmed we have got at least one chick on the nest," reserve manager Simon Clarke told the BBC.
It could mean establishment of Britain first great white egret colony, experts said, as clutch sizes are usually three or four eggs and adults tend to return to the same nest site if they successfully raise chicks there.
White egrets are found across the world, with an estimated population of 2 million, there are no records of them having bred in Britain.
"To have an amazing bird like a great white egret, which is the size of a grey heron [and] bright white, nesting here is just phenomenal," Clarke said.
Clarke said he is is hoping to confirm more chicks.
"We're just waiting to see how many more turn up."