Since Twitter introduced its trending list in 2008 it says it has faced near-constant questions along the lines of "why isn't X trending".
In 2010 it published a blog explaining why #wikileaks wasn't trending, and that it had nothing to do with Twitter blocking it or related topics appearing in the list.
Twitter Trends are automatically generated by an algorithm that attempts to identify topics that are being talked about more right now than they were previously. The Trends list is designed to help people discover the 'most breaking' breaking news from across the world, in real-time. The Trends list captures the hottest emerging topics, not just what's most popular.
So, #CameronMustGo hasn't been blocked, it just isn't the 'most breaking' breaking news.
In fact if Twitter didn't have such rules in place then #CameronMustGo would probably never have trended in the first place, as the list would be occupied by near constant references to One Direction or Beyoncé.
In 2010 Twitter actually had to block Justin Bieber from the trending list, following a fan-led campaign to have him there permanently.
i100.co.uk has reached out to Twitter for a comment on #CameronMustGo.