MI's 13 and 18, and their lack of official existance.

You've probably heard of MI5 and MI6.

There are both part of the Joint Intelligence Committee, and are basically our highest level intelligence and security services.

MI5, known these days as the Security Service, deals with things that happen in the UK. It protects the UK from things that happen in the UK. Think Spooks.

MI6, or the Secret Intelligence Service, protects the UK from things happening outside of the UK. Think James Bond.

Maybe, though, you didn't know that it goes up to MI19.
Of course, maybe you do/did know that, in which case it's probably not worth reading on.

The Military Intelligence (not Mission Impossible as some of you were hoping) departments were set up during the First World War basically to have a more effective and co-ordinated intelligence service. All the different departments had distinct purposes.

MI1 dealt with code-breaking.
MI2,3 and 4 all dealt with geographical information from different parts of the world.
MI7 dealt with the press and propaganda.
MI8 dealt with communications.
MI9 dealt with the aid of resistance fighters and British PoW's abroad, but also with interrogation of the enemy.
MI10 dealt with weapon and technology analyses.
MI11 dealt with military security.
MI12 was similar to MI7, but focused exclusively on censorship.
MI13 didn't exist, and we will look at that more specifically in just a minute.
MI14 dealt with information about Germany.
MI15 dealt with aerial defence.
MI16 dealt with scientific developments.
MI17 dealt with the secretarial tasks of all of the MI departments.
MI18 didn't officially exist, but it has rumoured to have dealt with Radio Security.
MI19 was set up later to fully replace MI9 at enemy interrogation.

See I was inspired to write this on the basis of my previous research around 13, and my intrigue was piqued by the lack of an MI13.

There isn't an obvious reason why it was left unoccupied. Even if its for superstitious reasons that doesn't really explain why they would leave MI18 open as well.

I have a few theories. I have no idea of how grounded these ideas might be.

Maybe MI13 and/or MI18 were originally intended to do something that they thought would be important, but ended up not being, so were unnecessary.

A possibility is that the government imagined that the details of these departments would never be made public, and as these departments could have been something they didn't want their allies, and even their own people to know about.
Of course this opens up the debate to much more interesting theories about what MI13 and MI18 could have been.

The most popular of these suggestions appear to be a department regarding UFO's, or advancd/alien technology in the war effort.
Perhaps the more sensible idea is that these departments were set up to monitor information about our own allies in case they should turn against us.

This seems much more plausable, but I would imagine at this point we will never ever know.