The 23-year-old was forced to cancel her gigs in Cardiff and Blackpool on September 7 and 8 because of a bad cold, and the Someone Like You singer is still ill.
In a blog, Adele wrote how she did attempt to continue the tour.
She said: 'I'm still not well enough. I woke up feeling a bit better but not by a lot, but decided to head towards Cardiff anyway in the hope I'd be able to sing tonight - half way there I realised it was unrealistic.'
The star added: 'I apologise profusely, I know many of you would have travelled to Cardiff today for the show. I'm so so sorry!'