Although hunting and the loss of habitat wiped out the crane population in Britain, the Great Crane Project has been rearing the birds in captivity and reintroducing them to southwestern England since 2010, they said.
The egg, the first laid by cranes released by the project, is under round-the-clock guard in a wetland near Slimbridge in Gloucestershire, the BBC reported Monday.
Video observation of the nesting birds has been set up to collect data as a resource for conservationists and also to protect the nest from egg collectors, project officials said.
"Cranes are an iconic part of British wildlife and one that was all but lost for centuries," Nigel Jarrett of the Wildfowl and Wetlands Trust said.
The oldest of the cranes released by the project only reached maturity this year.
"There is a long way to go before cranes become widespread again, but it is absolutely momentous to see this egg laid at Slimbridge," Jarrett said.