the lord of the rings - Where did "you shall not pass" come from?

One of the most memorable lines from the Lord of the Rings movies is when Gandalf stands before the Balrog and says "You shall not pass."

EDIT: Actually, he says "you cannot pass" in the movie too. It's just commonly quoted as "you shall not pass" because he says that later.

He doesn't actually say that in the book.

In the Fellowship of the Ring, Gandalf blocks the Balrog saying:

You cannot pass....I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass. The dark fire will not avail you, flame of Udûn. Go back to the Shadow! You cannot pass.

What is the reason for the shift?

Answer

It was most likely a conflation with “they shall not pass”, which the Wikipedia article notes:

was most famously used during the Battle of Verdun in World War I by French General Robert Nivelle. It appears on propaganda posters, such as that by Maurice Neumont after the Second Battle of the Marne, which was later adopted on uniform badges by units manning the Maginot Line. Later during the war, it also was used by Romanian soldiers during the Battle of Mărășești.

This opens the possibility that Tolkien himself was quite well aware of that form of the phrase and may have even been inspired by it.

It was also used during the Spanish Civil War and has since been used by various anti-fascist groups worldwide, as well as by the Sandinistas (according to the same Wikipedia article).

The timeline of it's usage, especially by the Sandinistas, makes it most likely that the conflation comes from the 60s counter-cultural movements, when their usage of it would have been well known and when popularity of LotR was really starting to take off (particularly in the US). Of course, someone who was around at the time would be needed to confirm that.