The TV journalist, 64, was first diagnosed with the illness in 2014 and later said it had spread to his liver and lymph nodes.
Speaking to The Times, Alagiah has now revealed doctors told him in April the cancer is now in his lungs.
He told the newspaper: “My doctors have never used the word ‘chronic’ or ‘cure’ about my cancer.
“They’ve never used the word ‘terminal’ either. I’ve always said to my oncologist, ‘Tell me when I need to sort my affairs out’, and he’s not told me that, but what he did tell me is that the cancer is now in a third organ. It is in my lungs.”
Alagiah, who tested positive for Covid-19 in March, said he had kept the latest development secret, only telling his editor.
He said: “I said to my doctor, ‘You’re going to have to do the worrying for me.’ I don’t want to fill my mind with worry. I just know that he’s a clever guy, doing everything he can.”
Alagiah said his chemotherapy has increased as a result of the cancer spreading.
Sri Lanka-born Alagiah was diagnosed with bowel cancer in April 2014 and again in December 2017.