The 82-year-old, who is most recognised for her role as Walford smoker Dot Branning, will take a short break from filming at the BBC soap to take on the role of Jessie, the show's Miss January, currently played by Sian Phillips.
Speaking to the Daily Mail, Calendar Girls producer David Pugh said: "It was the moment in the royal retiring room, with June sitting there with her cigarette and a glass of Champagne. Dafydd [Rogers, co-producer] came in just as she was lifting her dress to show me her legs - I have to tell you, they're a bloody good set of pins. They're the legs of a 42-year-old!"
He added: "She asked if she could be a little more brave in the part. Sian uses balls of wool and something that has been crocheted to cover her - so for June I'm thinking smaller balls of yarn and a slightly smaller piece of knitting."
As part of the new cast, Brown will appear alongside former Albert Square co-stars Anita Dobson as Chris (Miss October), Jill Halfpenny as Cora (Miss July) and Jack Ryder as the photographer, Lawrence.
Other members of the new lineup include Jerry Hall as Celia (Miss September), Jill Baker as Annie (Miss February), Sara Crowe as Ruth (Miss November) and former Hollyoaks actress Gemma Atkinson as Elaine.
The Calendar Girls story is based on genuine members of the Rylstone & District Women's Institute who stripped for a calendar photoshoot to raise money for leukaemia research.
Writer Tim Firth, who adapted the stage version from the original 2003 film, will "substantially rewrite" the script for the new cast.
A Walford spokeswoman this afternoon confirmed to DS: "June Brown will be taking time out from her EastEnders schedule to take part in the West End production of Calendar Girls. June will be returning to her role as Dot Branning after her theatre run is completed."