Dame Julie Walters has revealed she was diagnosed with stage three bowel cancer.

The actress, who has starred in Mamma Mia, Billy Elliot and Educating Rita, told the BBC’s Victoria Derbyshire she initially thought doctors “must have made a mistake”.

Having had chemotherapy, the actress has now been given the all clear.

She said her next film, The Secret Garden, could possibly be her last – although she is not certain to retire.

Dame Julie said she had been diagnosed with stage three bowel cancer – which means the cancer has spread to nearby lymph nodes but not to distant body parts – 18 months ago, with two primary tumours in her large intestine.

The 69-year-old said she had first gone to see her doctor a year earlier with indigestion and “slight discomfort”, and later returned with symptoms such as stomach pain, heartburn and vomiting.

She was then referred to a gastric surgeon, where she had a CT scan.

The actress had been on set filming The Secret Garden when she received a phone call asking her to come in. The specialist told her they had found an abnormality in her intestine, and feared it was cancer.

“I was still thinking, ‘That’s ridiculous, he must have made a mistake’. I couldn’t believe it,” Dame Julie said.

She remembers, still in shock, the moment she told her husband Grant Roffey the news.

“I’ll never forget his face. Tears came into his eyes.

Dame Julie said she had always maintained hope of a recovery, having been told by her doctor: “We can fix this.”

But she said there were moments – especially waiting for surgery – when she thought: “Well, I may not come round from the anaesthetic.”

Dame Julie explained she had “30cm taken out of my colon” in hospital.

On coming round from her anaesthetic, she said – still feeling its effects – she had had the “weird” experience of “feeling absolutely marvellous”.

“I said to the night nurse, ‘Is Love Island on?’ – because we were talking about it – and we watched it together.

“It was only a couple of days later I thought, I feel exhausted, and a bit low actually.”

After being initially reluctant, she opted for chemotherapy, which she said was “fine” and had not caused hair loss.

Smiling, she said she was now she was “really well”, adding: “I’ve just had a scan, and I know that [I’m] clear.”

Her recovery, however, had meant she had to be cut from certain scenes in the soon-to-be-released film The Secret Garden, in which she stars alongside Colin Firth.

She said she also missed the premiere of Mamma Mia 2, with her agent telling people she had a ruptured hernia so she could keep the diagnosis out of the spotlight.

Dame Julie told Derbyshire – who has previously documented her own recovery from cancer in a series of video diaries – the diagnosis had “completely changed” her perspective towards acting.

“The person before the operation is different to this person.”

She said it was in some ways a “huge relief” to get off the “merry-go-round” of starring in films and on television – which she said although she found enjoyable, was also stressful and consuming.

“I was due to do two big series… and there were two films. And I just didn’t have to do any of it. And that was wonderful.”

Asked if she thought The Secret Garden could be her last film, she said it was possible.

“It would have to be something I’m really engaged with [to take another role on].

“I’m not saying I’ll never act again. But I certainly don’t think I can go back to [a film that requires working] six days a week, five in the morning till seven o’clock at night.”

What are bowel cancer symptoms?

A persistent change in bowel habit – going more often, with looser stools and sometimes tummy pain

• Blood in the stools without other symptoms, such as piles
• Abdominal pain, discomfort or bloating always brought on by eating – sometimes resulting in a reduction in the amount of food eaten and weight loss

Most people with these symptoms do not have bowel cancer, but the NHS advice is to see your GP if you have one or more of the symptoms and they have persisted for more than four weeks.

And if you, or someone you know, have been affected by cancer, information and support is available on the BBC’s Action Line page.