From the official site:

With great regret, George Michael has been forced to postpone his concert tonight (Wednesday, October 26) at the Royal Albert Hall, London.

A doctor visited George this afternoon and confirmed he is suffering from a viral infection and has a high temperature, so on medical advice he has been advised that he cannot perform tonight to allow him time to recover before his next concert at the Royal Albert Hall on Friday, October 28.

George has performed 35 shows to date as part of his Symphonica – The Orchestral Tour. The tour commenced in late August and George had his first UK concert last night in London.

A spokesman for George Michael said that, “He is disappointed that he cannot perform tonight’s concert, but he has been advised to rest. He received a tremendous reaction from the audience last night and is very sorry that he has had to postpone”.

The promoters have asked ticket holders for tonight’s concert to retain their tickets.  Further announcements regarding a replacement date will follow.