|The Rt Hon William Towers MP|
|First seen||Series 9, Episode 1|
|Portrayer||Simon Russell Beale|
A shrewd and honest politician, with a strong sense of humour, he succeeds Andrew Lawrence as permanent Home Secretary after the latter's death. He understands the necessity of MI5 and the things they need to do to keep Britain safe. He very rarely questions Harry or what he claims needs to be done and gives him as much slack as necessary and possible. The clearest exemple of this is in Series 9, episode 4 when Harry's actions could jeapordize an important deal between the US and the UK on desalination technology and Towers tells him that he doesn't want to lose it which is why '[they] can afford to piss them off only a little longer.'
He is also shown as Harry's strongest ally outside MI5. Towers was the one to stop Harry's tribunal in episode 1 of Series 10; he later regretted not being able to get him the full pardon. He also gave Harry his unconditional support after Ruth's death, whether he decided to remain or leave the service.