Does anyone know what M15 means in the following context?
"And we are again at war albeit with an unseen enemy whose poisonous tentacles have the potential to reach into every corner of our world. At the time of writing, according to M15, the current UK threat level for international terrorism is "severe" – which means that an attack is likely."
The paragraph above is from this article: http://www.telegraph.co.uk/news/uknews/terrorism-in-the-uk/12109626/Its-time-to-bring-back-national-ID-cards.html In case you need more context I can't find anything useful on the internet. Thanks!