...and remains loyal.
Speaking on his way to Perth, Australia, for a Commonwealth summit, the Prime Minister also warned against the 17 Eurozone nations colluding to undermine the European Union's free market rules.
He said: "London is the centre of financial services in Europe. It's under constant attack through Brussels directives. It's an area of concern, it's a key national interest that we need to defend."
Mr Cameron said that he will fight to prevent closer integration of the Eurozone countries leading to anti-competitive regulations.
"As the 27 we need to make sure that the single market is adequately looked after."
"There are a lot of things the eurozone is doing together. Having more meetings alone, establishing machinery - it raises the question of could there be caucasing?"