The star, who is 70 tomorrow, revealed that she had surgery and chemotherapy after doctors found two primary tumours in her large intestine.
Dame Julie received the shocking diagnosis while filming The Secret Garden, the new adaptation of Frances Hodgson Burnett’s classic novel.
Suffering from indigestion and discomfort in her side, she visited her GP and tests found stage three bowel cancer, which indicated the disease had spread into nearby lymph nodes.
Dame Julie said yesterday: “I was still thinking, ‘That’s ridiculous, he must have made a mistake’. I couldn’t believe it.”
Recalling the moment she broke the news to husband Grant Roffey, she told the BBC’s Victoria Derbyshire show: “I’ll never forget his face. Tears came into his eyes.
“I went into hospital and had a foot taken out of my colon. The brilliant thing about it was that it was in my bowel and if you catch colon cancer early, it’s one of the best cancers you can have.”
Asked whether she feared that her luck had run out, the Educating Rita and Billy Elliot star said: “I thought it could run out, especially waiting for the operation. I thought, ‘Well I might not come around from the anaesthetic’. That’s a big worry, isn’t it?”
While she was recovering, some of Dame Julie’s scenes in The Secret Garden, in which she plays Mrs Medlock, had to be cut.
She also missed the premiere of the 2018 film Mamma Mia! Here We Go Again, in which she played Rosie. To keep her diagnosis a secret, her agent said she had a ruptured hernia.
Dame Julie said of the subterfuge: “I wasn’t ready to talk about it.
“It’s too tender and too private and I just wasn’t ready.”
But the actress said it was a huge relief when the diagnosis forced her to “get off this merry-go-round”.
She said: “It felt like I was stepping off something and it was wonderful.
“I was due to do two big series and two films and I just didn’t have to do any of it.”
Dame Julie now says The Secret Garden, due in cinemas in April, might be her last film.
She thinks any new project would need to be something special to lure her back and involve a much less intense schedule.
The star admitted: “It’s the way I approach acting that isn’t probably healthy.
“I love it, it’s an exciting sort of stress, but it’s stressful.” Dame Julie said the death of Love Island TV host Caroline Flack showed how actors and other celebrities could be affected by public opinion in the age of social media.
She said: “If you’re slightly fragile and you’ve got a wall of that coming at you, it’s unhealthy, it makes people ill. Poor old Caroline Flack is a big example of that.” However, Dame Julie has no worries about turning 70 tomorrow.
She said: “People will perceive it as old, but I don’t.”
And you urged anyone with unusual bowel symptoms to go and have them investigated.
She added: “You’ve got to go and get things checked” then joked: “Doctors are used to bottoms. They’ve got one themselves. Hopefully.”
Genevieve Edwards, of Bowel Cancer UK, praised her for speaking out about her diagnosis.
She said: “It’s only by talking publicly about this disease and raising awareness that we can encourage more people to take action if they have concerns.
“Every year in the UK nearly 42,000 people are diagnosed with bowel cancer, making it the UK’s fourth most cancer killer.”
NHS England’s clinical director Professor Peter Johnson said: “I would like to thank Dame Julie for sharing her story.
“If caught early enough, bowel cancer can be cured. Going to your GP and getting checked as soon as you have symptoms is crucial.”