Olivia Williams will "never" be cancer-free

She told The Times: "I go in [for scans] like a puppy with this optimistic, bright face and then they give me bad news and it's like, oh my God, I fell for it again.

"They've found new metastases pretty well either just before Christmas or in the middle of a summer holiday. Then, for three years in a row, they started appearing too close to major blood vessels to zap. So there was a period when we were just sitting and watching them grow, which is a horrible feeling.

"I go to a room in King's College hospital and people in hazmat suits come in with a lead box of a radioactive material, which they inject into me and I become radioactive.

"It's supposed to buy me maybe a year, maybe two or three years, of freedom from treatment. In the best-case scenario it would have made [the metastases] disappear but that didn't happen.

"If someone had f****** well diagnosed me in the four years I'd been saying I was ill, when they told me I was menopausal or had irritable bowel syndrome or [was] crazy - I used that word advisedly because one doctor referred me for a psychiatric assessment - then one operation possibly could have cleared the whole thing and I could describe myself as cancer-free, which I cannot now ever be!"

The actress is now an ambassador for the London Marathon's charity of the year Pancreatic Cancer UK and insisted that "change" is possible, but it is "too late" for her.

She said: "The average time from diagnosis to death is three months - and that figure has not improved in 50 years.

"It takes an average person with my cancer [a VIPoma cancer within the pancreas] 11 visits to the GP to be diagnosed. For me it was probably about 21 times.

"Because it's so quick and so shocking, people tend to liken losing someone to this cancer to losing them in a car crash.

"What could change that is early detection with a test that could be as simple as breathing into a bag at your GP. We're incredibly close, we just need to get it over the line.

"It's too late for me, and for all these people who are running in the marathon who've lost a parent or a friend, who could have been saved by a pancreatectomy [removal of the pancreas] if the cancer had been found before it spread.

"This is where I get emotional but I'm not looking for sympathy, I'm looking for a cheap, early test."