How to address a knighted academic?

The canonical reference for this subject is Debrett's (via the Wayback Machine). They recommend that a letter to a knight should begin "Dear Sir [given name]," regardless of whether they have other titles such as "professor".

In fact, they advise against mixing titles granted by the sovereign (such as knighthoods and ranks in the armed forces) with titles not granted by the sovereign (such as professor) but, as others have observed, "Professor Sir [Given name] [Surname]" is common usage.

In an academic context, your alternative option for a salutation is "Dear Professor [Surname]". However, as I noted in a comment to the question, it's unlikely that somebody who's the CEO of a multinational company would, in fact, be a professor.

Within my university, knighted professors tend to use the Professor Sir {calling name} {family name} form as given by EnergyNumbers but I think that it is down to personal preference.

Just to add that the Sir / Dame is associated with the first name while Professor / Dr is associated with the surname. As you asked about a letter, if you would normally put 'Dear Professor {surname}' then that would still be correct. In order to use Sir / Dame you would need to also include the first name, or only use the first name.

Some of the answers and comments above are just plain wrong. In my professional career in the UK I have written hundreds of official letters from the highest branches of the UK government to academics who have been knighted. The correct form of salutation to a knighted professor, with the sole exception of a letter to one who is also a personal friend, is: 'Dear Sir [given name]'.

It is a quite separate question as to how such a person should be described, for example on the address of the letter. Whatever Debrett's might say, the predominant practice is "Professor Sir [given name][family name]".

There are various types of knighthood and it would be discourteous in a formal letter to omit such post-nominals as apply. So, for example, a letter to the late Peter Swinnerton-Dyer would have been addressed to "Professor Sir Peter Swinnerton-Dyer Bart." after he inherited the baronetcy in 1975, but after he was knighted for his own services in 1987 you would need to add 'KBE' after the 'Bart.'

If anyone reading this thinks that the previous paragraph is ridiculously pedantic, I can only say that experience has taught me that some of the recipients of knighthoods are very proud of their honours and take offence if they are not properly recognised. If you are writing to a knighted professor you do not know, it is wiser to err on the side of formality until you are told clearly that he does not care about such things.