The actor who played Robbie Rotten in popular kids' TV show has revealed he has terminal cancer.

Stefan Karl Stefansson told fans to 'dream big' as he took to Twitter to announce his devastation as the disease returned, telling his followers that it's inoperable.

He's best known for playing the villain in the iconic children's show.

He wrote in an emotional statement: "It’s not until they tell you you’re going to die soon that you realize how short life is. Time is the most valuable thing in life because it never comes back. And whether you spend it in the arms of a loved one or alone in a prison-cell, life is what you make of it. Dream big."

(Image: Instagram) (Image: Twitter)

A fan quickly asked: "Wait.....It's BACK!?"

And Stefan, 42, responded: "Unfortunately it's back and this time they can't operate on it."

The married star, who has three children, later added: "Yesterday, no one could tell what would happen today, only that today might come. Life is like that every minute, we are waiting for it, will it come or will it not, but why? We should always life now because now has come, life is here and only now, so go get it!"

He was inundated with support from his devastated fans.

"Stay strong," one told him.

While another said: "All my love goes out to you and your family. Make the time you got left count"

(Image: BBC)

And another commented: "We cant lose another legend"

Stefan was diagnosed with pancreatic cancer in 2016 and later had surgery on his liver.

He's been open about his battle with the disease, regularly sharing pictures of himself in hospital.

His wife, actress Steinunn Ólina Þorsteinsdóttir also posted a statement about her husband's diagnosis.

She said: "Even though I knew for a long time that this day would come, it's a difficult turning point.

(Image: Instagram) (Image: Instagram)

"There is no cure for this. Now he will have to undergo chemotherapy to try to prolong his life and we hope that this will go as well is possibly can, to improve how he feels and to prolong his life."

She added: "His illness has had an impact on the whole family, naturally.

"Some days we feel very low, some days we are happy and are happy about all that we have accomplished together and lived together.

"Nobody can take that away from us, however this goes. Not even death itself."