The actor, who plays Bill Webster, will leave the show after being told by producer Phil Collinson that there was no future for his character.
"Bill is off travelling with girlfriend Pam in the story. Peter wanted to know where the character was heading as there was talk of him becoming seriously ill," a source told the News of the World.
"He was really surprised when Phil told him Bill wouldn't be coming back. The cast and crew were shocked as Peter is a popular guy. But that was it.
"Peter understands the fickle nature of TV. But people like Bill. He hasn't been killed off so the door is open for him to return one day."
Armitage first appeared in the soap for a year in 1984. He returned ten years later before being written out in 1997 when Bill ran off to Germany with Fred Elliott's wife Maureen. He returned for his current stint in 2007.