Kenny Elrick / Rex Features
Rustie, 54, has signed up to play Opal, the fiery aunt of market sweeper Gus Smith. Gus, played by Mo George, is being written out of the soap by "mutual consent".
"I'm absolutely over the moon to think I'm going to be in the Square," Rustie said. "I can't wait."
Lee made her name by appearing on breakfast station TV-am in the 1980s. She later went on to present ITV gameshow Game for a Laugh.
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