Richard Bacon is fighting for his life after being put in a medically-induced coma.
The 42-year-old radio presenter recently revealed he was suffering from pneumonia across both lungs, but is now said to have contracted a mystery respiratory virus and has been in a critical condition for a week.
A source close to the former 'Blue Peter' presenter said: "This is an extremely worrying time for everyone who loves him.
"Doctors started treating him for pneumonia but it soon became apparent that it wasn't working and his condition took a dramatic turn for the worse."
Doctors are said to be "no closer" to finding out what is wrong with the star.
The source added to The Sun newspaper: "They had no choice but to put him into a medically induced coma to save his life because things spiralled so fast, but that was a week ago and they are no closer to finding out what is wrong with him or how to help him.
"It's touch and go, and extremely upsetting for everyone who loves him. They are beside themselves with worry."
Richard recently revealed he was taken off a plane and put in a wheelchair before being rushed to hospital, and was later diagnosed with double pneumonia, an inflammatory condition of the lungs.
He wrote on Twitter: "Thank God I got ill in Britain (actually on the way to Britain, was taken off the plane in a wheelchair). F**k all the ideology driven politicians who've messed up America's healthcare system. Viva the NHS. Happy 70th. (sic)"
In response to BBC News presenter Rachael Bland asking if he is OK, the former 'Blue Peter' presenter replied: "Basically pneumonia. But across two lungs. I'll be here a few days. (sic)"