The builder and TV personality, best known for hosting the BBC’s garden makeover show from 1997 to 2005, shared the news with audience members at a charity event last November.
News of the diagnosis emerged after footage from his presentation at The Swallows Head & Neck Cancer charity was posted on YouTube.
The 67-year-old presenter said doctors had discovered a “three-centimetre tumour in the lung” and that he had an emergency appointment for a CT scan scheduled in London the following day.
Walsh had not provided any further updates about the results of his follow-up scan at the time of writing.
Speaking at the conference on 27 November, he said: “I have an emergency appointment at 9am tomorrow morning back in London. I had a chest infection, had to go to the doctor and they sent me up to the hospital for a CT scan and they think there’s a three-centimetre tumour in the lung.”
“I’ve tried to keep cheerful and you’ve cheered me up today, and thank you for letting me reminisce and talk. I was weighing up whether or not to tell you about this but I felt it would be unfair if I was to leave this and I didn’t tell you.”
“That’s why I’m going to have to leave after this talk and I won’t be able to join you for a few beers tonight,” he added.
The Independent has contacted Walsh’s representatives for comment.
Walsh discovered two benign lumps on his chest in 2002 and underwent surgery to have them removed.
The former Homes Under the Hammer host has a family history of the disease; his sister reportedly underwent a lumpectomy to remove cancerous breast tissue when she was 31.
In a 2013 interview with The Mirror, he said the discovery was a wake-up call for his health.
Walsh explained: “I just buried myself in my work and then went along for the day surgery where the lumps were removed under a general anaesthetic. It was only then that it sort of hit me that this could actually be serious and I could have breast cancer, like my relatives.”
Two decades after his first scare, the father-of-three learnt he had a lump in his throat during a visit to the GP for an unrelated issue.
“I went to my GP about an unrelated issue and while I was there, he discovered this lump in my throat that he was a bit concerned about. He said it may be nothing but that he would rather I went and got it checked and, unfortunately, it turned out to be cancerous.
“I had to go in straight away and have surgery to remove it. I am now fully recovered and even back playing football!” he said in June last year.