His message, coming from a man whose appointment No 10 fiercely opposed, will be a bitter blow for Cameron, who has hinted at wanting to change the rules surrounding the principle of European free movement to curb migration.
At least Cameron can count on the public's support in his fight, or can he? A poll by ComRes for the Independent on Sunday over the weekend found that most Brits (52%) want to be free to live and work "anywhere in the European Union".
However, we aren't quite so warm about letting our fellow Europeans have the same right to live and work in Britain, with barely one in three (36%) supporting it, compared to the 46% who hated the idea.
The contradiction runs deeper, as voters get in a muddle over the implications of European free movement.
Ukip voters are not so sure how to react. A whopping majority (75%) disagree with letting other Europeans live and work in Britain, while 41% of them disagree with Britons having the right to do the same in the EU. Just to make things even clearer, a similar amount of Ukip supporters (37%) agree with Britons being able to live and work "anywhere" in the European Union.
Conservative voters are just as confused though, with 51% of those surveyed saying they oppose the right for other Europeans to live and work in Britain, while 49% say Britons should be able to do exactly that anywhere they like in Europe.
The message for Cameron is mired in contradictions. Voters want to curb the number of Europeans coming to Britain, but to freely go anywhere they like in Europe.