He described how his coughing and shortness of breath became "incredibly severe", until his coughing brought up a "solid object".
"This morning I am relying on NO external oxygen to breath at all and I'm feeling bloody fantastic," he said. "The doctors have discussed what's happened and the only plausible conclusion is that I've literally coughed up a tumour."
Stephen has consistently repeated that his condition is terminal, and said today that it is still a case of counting the days - adding that "at the moment the days just keep on coming!"
Such is the support for Stephen online that his latest update has already been "liked" by more than 100,000 people.
Cumberbatch and fellow actor Simon Pegg have sent messages of encouragement via Twitter, urging people to donate to Stephen's cause, and last night Simon Cowell pledged to make a "significant" contribution himself.
Important, lovely message that Benedict has asked me to pass on for @_StephensStory Text Stephen to 70500 to give £5. pic.twitter.com/c4M9b74PIU
Stephen, from Burntwood, Staffordshire, said the reaction to his cause had confirmed his belief that people are "good" and called everyone's response "heartwarming".
He wrote: "I always describe my cancer as a huge kick up the backside that taught me a lot of good: the importance of helping others, of making every second count, of making sure life is for LIVING."
- UK Independent