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The actress, who played paramedic Nina Farr in the BBC medical drama until 2006, underwent surgery on Wednesday to have a lump removed.
She first noticed it while seven months pregnant with daughter Gigi, who was born 11 weeks ago.
She wrote on her Mirror blog before the surgery: "By the time you read this, I will be lying on an operating table as surgeons try to cut out a lump from my breast.
"It's a fairly small lump really, about the size of a walnut, but it is doing a great job of trying to screw up my life at the moment."
The 35-year-old revealed that she was given her final diagnosis last week, writing: "First a male doctor asked me gravely: 'What do you know about cancer? I think you've got it.' Two days later a cancer specialist nurse told me gently that his hunch had been right.
"She didn't use the word aggressive, but I know that it is. I don't need to look up statistics on the internet to know I may not make it."
She added that she had saved some mementos for her daughter while she was still pregnant, saying: "I packed up boxes of photos, keepsakes and memories as a legacy for my unborn baby, I wanted to have my things in order. I just didn't imagine I'd need to do it so soon."
