The Bee Gees musician was due to meet the prime minister yesterday (October 24) to talk about a cover of the band's track 'I've Gotta Get a Message to You' for the Poppy Appeal.
However, he made an appearance on The Alan Titchmarsh Show with The Soldiers to perform and talk about the appeal.
"Robin is not well," a friend of the star told The Mirror. "But he has always been a trouper and is determined to fight this. He is a very brave man indeed."
Gibb cancelled a planned visit to the US and Brazil in order to receive treatment for stomach pains in April. He was admitted again for similar treatment earlier this month.
The musician also had surgery in 2010 to clear a blockage in his intestine.
Watch the video for The Soldiers and Robin Gibb's 'I've Gotta Get a Message to You' below: