Dame Esther Rantzen has been diagnosed with lung cancer.
The 82-year-old broadcaster says she is having tests to assess the best treatment for her condition but is feeling "optimistic", even though the disease has "spread".
Esther said in a statement: "In the last few weeks I have discovered that I am suffering from lung cancer which has now spread.
"At the age of 82, this diagnosis has prompted me to look back over the years, and I want to express my profound thanks to everyone who has made my life so joyful, filled with fun, and with inspiration.
"First and foremost my family. My three children Miriam, Rebecca and Joshua have been the most wonderful support, company, and source of love and laughter and I am deeply grateful to them.
"My friends have been amazing and have created memories which sustain me and give me strength.
"My colleagues with whom I have worked, and continue to work with in broadcasting, journalism, the voluntary sector, and in many other organisations, have been a constant pleasure, and have amazed me with their tolerance of my wild ideas and awful jokes."
She continued: "I have been continuously inspired by the courageous children, older people and viewers who have trusted me with their life stories. I have always tried to live up to that trust.
"As I am sure you will understand, while I am awaiting the results of the tests, I am unable to answer questions.
"Thanks to the extraordinary skills of the medical profession there are wonderful new treatments, so I am remaining optimistic."
The star was a trailblazer for female broadcasters and became a household name with her career on the BBC, where she notably presented the consumer rights programme 'That's Life' for 21 years.
Esther also founded the charity ChildLine, a support charity for children and young people in the UK, in 1986 before setting up The Silver Line - a charity for elderly people struggling to cope with loneliness - in 2013.
She was made a Dame in 2015 for her services to children and older people.